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Vern Ehlers (R-MI), first elected in 1993
Ph.D. in nuclear physics from UC Berkeley; former chair of the Physics Department at Calvin College in Grand Rapids, MI.

Rush Holt (D-NJ), first elected in 1998
Ph.D in physics from NYU; assistant director of the Princeton Plasma Physics Lab 1989-1998.

Nancy Boyda (D-KS), first elected in 2006
Chemistry/Education double major in college, then worked as analytical chemist and field inspector for the EPA.

For much of the past decade, I believe Holt and Ehlers were the only scientists in Congress; they jokingly referred to themselves as "the bipartisan physics caucus". Boyda's election made it three. And soon, there may be a fourth: Bill Foster is running for Dennis Hastert's seat in Illinois. His science background includes a Ph.D. from Harvard, and 20 years at Fermi National Laboratory (aka "Fermilab"), and he just got endorsed by 19 Nobel laureates.

If he gets elected, the number of scientists in Congress will have doubled in just a couple of years :)
Interestingly, the 4 of them would be 75% Democrats, and 75% physicists; also, the one woman is the only non-physicist. And we still wouldn't have a biologist. (Anyone wanna draft

fyfer for Congress?)

Edit: Thanks to my posting this on Blue Mass Group I discovered that there's one more scientist in Congress who I totally missed, despite a) having paid attention to who the scientists in Congress were for years, and b) having met this Congressman and chatted with him in person. And even a thread on dailykos about scientists in Congress didn't catch it. Here's one more:

John Olver (MA-01), first elected in 1991
Ph.D. in chemistry from MIT; professor of chemistry at several universities, including MIT and UMass-Amherst.

... still no biologists :)