The recessive lethal T/t-complex haplotype tw75 was isolated from Jena, East Germany, prior to 1973. It was at first thought to be a standard member of the tw5 complementation group because matings of T/tw75 × T/tw5 produced tailless offspring only (Dunn, Bennett & Cookingham, 1973). Cross tests with all other complementation groups and embryological studies have recently been completed, with results demonstrating that tw75 does not complement the members of either the tw5 or the tw1 complementation groups. Histological studies show that compound embryos tw75/tw5 and tw75/tw1 die with symptoms indistinguishable from tw5/tw5 and tw1/tw1 homozygtes respectively. Thus tw75 represents the first confirmed isolation of a t-haplotype that overlaps two complementation groups. © 1979, Cambridge University Press. All rights reserved.