A chromosome study was performed on 2 sympatric species of marine fish,Fundulus heteroclitusL. andF. majalis. A modification of techniques which included the combination of the procedures of McPhail&Jones (1966) with those more recently developed by Ohnuki (1968), for demonstrating the spiral structure of cultured human leukocytes, resulted in clear and distinct chromosome spreads from gill epithelial cells of these 2 species. Optimum chromosome spreading and accumulation of metaphase figures resulted when specimens ofF. heteroclitusreceived either a 0·28 ml or a 0·4 ml injection of a 0·1 % colcemid solution for 45 h and 48 h respectively. Best results inF. majalisoccurred when specimens received a 0·1 ml dose of the 0·1 % colcemid solution for 22 1/2 h. Studies of the karyotypes of 18 specimens ofF. heteroclitusand 9 specimens ofF. majalisindicated that the 2Nchromosome number for each species wa