ONE OF THE highlights of any Harvard Krokodiloe concert comes with the group's rendition of Cole Porter's "Let's Do It." The Kroks, the 12-man cappella undergraduate singing group, always revises the lyrics of this randy war-horse to give it that unmistakable aroma of burning rubber--the smell of tires and gonads hot for another trip to Wellesley.

On the particular night when the Kroks recorded their latest album last spring, one version of the retrain sent the packed Sanders Theater crowd into paroxysms of delight, the longest and loudest laugh of the night. Their tuxedoed frames neatly outlined against the rich wood paneling, they sang "In Tommy's Lunch, Eurofags Do It," and the place went wild.

The joke is funny an allusion to a certain social subset extant on the Harvard campus, known for their continental accents, upraised and narrow jacket collars, predilection for Neosynepherine-substitute, and tendency to gather at a seedy late-night luncheonette near Adams House. And the joke--and especially the reaction to it--reveals several truths about the Kroks and their appeal.

The Kroks are Preppies. Individually, they may not have spent their formative years in the celebrated sextet of boarding schools, but as a whole the Kroks embody the cultural effluvium of those institutions. They have that easy-to-identify, difficult-to-define social swagger that marks them instantly as embodiments of the best and worst of the Preppie zeitgeist.

One of the more neglected aspects of Preppiedom--generally ignored in favor of the more obvious signs such as indulgence in alcohol, snobbery, ritual sex and clothes of peculiar colors--is a reverence for achievement, particularly success that appears to come effortlessly. That is where the Kroks come in. Their performances seem to have two purposes; in order of importance, they are: 1. to show that they are having a good time, and 2. to display their singing talent. That the Kroks are indeed excellent singers comes almost as an afterthought; their primary message seems always to be, "This is what we like to do." Totally.

And this joy is infectious, and, combined with their refined style often makes for superb concerts. A cappella singing is an acquired taste, but the Kroks' impeccable song selection (mostly Cole Porter, Irving Berlin and Richard Rodgers stories of love and destruction), pleasing harmonies and prissily risque patter, all in the sensuous acoustics of Sanders create an enticing package. The Kroks don't just sing, they perform, with that Preppie obsession about sex used occasionally to roaring success, and never, in my experience, to disappointment.

The standard pattern goes like this: one member steps forward and introduces the next number with a few words of smirking humour (a word the Kroks almost demand to be spelled with a "u." The American way would seem somehow inappropriate). An example of the patter from their record:

Our next song is going to mark a departure from the usual repitoire (A word pronounced with extreme nasal intonation--intentional? I have no idea) of college men's chorouses. It's usually the unrelieved round of drinking songs and love songs and songs about girls and, well let's face it, sex. (minor laughter) This next song is going to cover a topic that is often neglected by groups of our kind; and that is (pause) horticulture.

And the song, of course, is "When you wore a tulip," in the standard '40s boy-meets-girl mode. So it goes, the five-part harmonies and do-wahs piercing the night air with a jovial and slightly condescending good humour.

But that is in person, and the Kroks present a curiously different picture on this, the group's 12th album over the years. Most songs are live, with occasional numbers done in a studio. The name "at Symphony Hall" comes from the liner photo that shows them at the Symphony subway stop. Unfortunately, the album serves to highlight the importance of the Krok's appearance, mannerisms and clever choreography to their overall effect. The harsh scrutiny of vinyl embellishes imperfections in the Kroks's non-professional voices, flaws that go unnoticed in the excitement of a live performance. For example, the group's version of "Blue Moon," an elaborately arranged scat number that never fails in concert, seems lifeless and stale without the little drama that accompanies it on stage. Likewise, "The Masochism Tango," one of their weaker numbers live, comes across childish and definitely unfunny on the record. There are exceptions, particularly Grant Bue's gutsy baritone solo on "What's Your Name?," and the last song on the album, a perfectly paced "Serenade in Blue," with an unusual and pleasing solo by tenor Steve Zelinger.

Porter's "Let's Do It", however, does work as well as it did in concert, probably because the song lives (and dies) by its inventive lyrics, e.g. "Catherine Deneuve with her Chanel does it and the fragrance really lingers/Colonel Sanders with his chicken does it, and then, he licks his fingers." Musically, both the live and to a lesser extent, the studio versions of the songs suffer from fairly primitive mixings, and the sound is frequently muddy. The Kroks draw inspiration, not to mention an occasional arrangement, from Manhattan Transfer, but can't match the polish or studio values that four-person group obtains in their albums. Only the most passionate Krok devotee (and they do exist) could draw nearly the same satisfaction from their record as from a performance.

But in concert, noses tilted ever so slightly skyward, vowels stretched to aristocratic dimension, the Kroks still satiate the regulars. (They play Lowell House tonight at 8:15.) For others, repulsed either by the endless references to Bermuda, the heavy Porcellian-Krok alliance, or by a cappella singing in general, the Krok music and spirit remains anathema. Yet the Kroks should be seen, and their unique talent appreciated, if not savored. Watching those immaculate dandies preen, one can imagine, just for a moment, how the other half lives, sings--and sometimes--flourishes.