SUMMARYThe genetical and developmental aspects of six dumpy mutants ofDrosophila melanogasterhave been investigated. The mutantsobm,lmandolv(possibly alleles),lv, lv1andlvI(possibly alleles) were known to be lethal when homozygous. Previously the lethal effect has been treated as a uniform effect. However, the lethal stage is not the same for all homozygotes, being egg/larval (E/L) for the threelvalleles, egg (E) forolv, larval/larval ecdysis (L/L) forlmand E/L and larval (L) forobm. Not all heterozygous combinations are lethal, i.e.obm/lmandobm/lvIare not lethal. Phenotypically the lethal heterozygotes fall into two patterns: (i) combinations not involving the alleleobmand (ii) combinations involvingobm. In the former, the mutant with the developmentally later expression is ‘dominant’ to the mutant with the developmentally earlier expression. In the latter, the genotypes manifest different proportions of individuals at the lethal stages E, E/L and L. Previous observations suggested that the lethality of the homozygoteobm/obmwas associated with the presence of an independent lethal in the stock. Observations presented here suggest that the lethality is a function of theobmallele itself. Complementation between some of the lethal mutants is not in accordance with the general rule for dumpy that compounds manifest the traits they have in common.