Theories of sympatric speciation have a long and controversial history (Mayr, 1963) and have enjoyed a resurgence of interest in recent years (e.g., Maynard Smith, 1966; Bush, 1975; Tauber and Tauber, 1977; Rosenzweig, 1978). Of particular interest is the role of behavioral isolation, which has generally been considered to evolve along with divergence in allopatric populations or by selection against hybrids after secondary contact. It has also been suggested that behavioral isolation can precede the development of postmating isolation (Bush, 1975; Tauber and Tauber, 1977). In this case, genetically simple behavioral differences might begin the process of genetic divergence of sympatric populations. The purported examples of this mechanism, however, have been criticized (Futuyma and Mayer, 1980; Henry, 1985), and there is no general agreement about the importance of sympatric speciation. Gittenberger (1988) has proposed a model of sympatric speciation, resulting from reproductive isolation due to differences in the direction of shell-coiling in snails. In the species that have been examined, the direction of coiling is controlled by a single pair of alleles. The expression of the genotype, however, is delayed by one generation, because the phenotype is determined by the mother's genotype. Gittenberger cites evidence from several species that snails with opposite coiling have difficulty in mating. He proposes that the consequent assortative mating could lead to sympatric speciation. Given the controversy surrounding sympatric speciation, it is important to evaluate such models carefully. The purpose of this note is to examine the properties of polymorphism for coiling that relate to sympatric speciation. Our analysis will focus on Partula suturalis, one of the few species of snails that is polymorphic for the direction of coiling and the only one to be studied extensively. Mating between snails of opposite coil is difficult (Lipton and Murray, 1979), and individuals without enough potential mates of the same coil produce fewer offspring (Johnson, 1982). This generates positive frequency-dependent selection (a disadvantage of rare types) and leads to monomorphism within populations. In nature, polymorphic populations are only found in steep clines between areas that are uniformly sinistral and dextral. The dextral populations of P. suturalis occur in the presence of sympatric and closely related sinistral species. The difference in coil has probably come about by selection against hybridization, favoring enhanced premating isolation (Clarke and Murray, 1969). The evidence for such selection raises the question of whether the variation in coiling could lead to genetic subdivision and speciation within P. suturalis itself.