SUMMARYThe genetic properties of some new mutantt-haplotypes derived from the naturally occurring haplotypet6have been investigated. Several mutant haplotypes gave taillessness with brachyury,T, were viable when homozygous, and when in compound witht6, and all these expressedtfand appeared to have arisen by meiotic crossing-over betweenTandtfin the region of crossover-suppression oft6. The haplotypeth17arose by crossing-over in a stock oft6carrying the translocation T(1; 17) 190Ca, in which the translocation break in chromosome 17 is within the crossover-suppressing region; henceth17could not be separated from T190. It was of the complementary crossover type in that it did not interact withT, and was lethal in compound witht6; its male segregation ratio was normal with heterogeneity. A further haplotype,th20, expressedtf, gave taillessness withT, and was lethal when homozygous and in compound witht6. The expression oftfwas attributed to a deletion, which also covered the loci of the nearby lethals knobbly,Kb, andtw5.th20showed weak complementation oftw5. The haplotypeth7had previously been interpreted as carrying a duplication; it was shown that this duplication was semi-lethal when homozygous. it is suggested that lethal mutantt-haplotypes commonly arise by duplication or deletion but such structural changes are not necessarily at the end of the haplotype, and need not arise by unequal meiotic crossing-over.