Analyses of crossover derivatives from certain heterozygotes carrying the Ab "allele" indicate that the separable elements of Ab reside in adjacent members of a duplication. The occurrence, in association with crossing over, of more than one type of derivative from each of two such heterozygotes is taken as evidence of variable local synaptic patterns reflecting homol-ogy between the A b members. The isolation of the alpha or proximal component from homozygous A b individuals in association with crossing over is explained on the basis of exchanges within obliquely synapsed members. This interpretation requires that the A b complex is an adjacent, serial duplication. Evidence for structural and functional variability among A b complexes of different geographic origin is presented. J. R. Laughnan.