Balancing selection may be invoked to explain the uniform frequencies of alleles at moderately and highly polymorphic allozymel oci (G3p1, Zdh-1, Ldh-3 and Pgml) , but is unlikely to account for the invariant patterns of alleles at weakly polymorphic loci. The proposed maintenance of such uni- form frequencies by selection contrasts sharply with the strong associations of several well characterized allo- zyme polymorphisms with varying environmental fac- tors such as temperature (e.g., VICUE and JOHNSON 1973; WATT 1977; PLACE and POWERS 1979) or salinity (e.g., KOEHN et al. 1980).