Collections from natural populations of Drosophila melanogaster have been analysed with respect to a sequence length polymorphism (delta-1 located within the first intron of the distal transcriptional unit of the ADH region) and to its allozyme polymorphism. Among 168 lines from northern Maine (Portland) and southern Florida (Tampa), a significant linkage disequilibrium (P<0.001) has been found between the ▽-1 and allozyme polymorphism. 49 lines (=0.293) contained allozyme F-▽1 F; 106 lines (=0.635), allozyme S-▽1 S; 9 lines (=0.054) allozyme F-▽1 S; and 3 lines (0.018) allozyme S-▽1 F. 42 lines of the second chromosomes including the alcohol dehydrogenase structural gene were estimated for ADH activity, total protein, and specific activities (the ratio of activity units to total protein per fly). This study shows that the ▽-1 and allozymes have an effect on the level of ADH specific activities. Analyses of variance for the specific activity, by using general linear models for factorial experiments, indicated significant variation of allozyme classes (P<0.01), that of the ▽-1(P<0.01), and interaction variation of lines (allozyme x ▽-1) (P<0.00001). These results indicate a possible role of the multisite or epistatic selection in understanding the evolutionary dynamics of the individual gene.