Reproductive isolation is demonstrated between morphologically distinct, sympatric demes of four epipelic diatom species inhabiting a small pond. The six demes ofSellaphora pupula, four of them allogamous, one probably autogamous, and one apparently asexual, differ morphologically with respect to size, shape, stria density and pattern. In the pond,Caloneis silicula, Cymatopleura solea, andNeidium ampliatum each contain two allogamous demes, differing principally in size. There are intrinsic barriers to hybridization between the demes, which operate at the earliest stages of sexual reproduction. Such gamodemes are perhaps best treated taxonomically as separate species.