Heterozygotes display abnormal indirect flight muscle (IFM) myofibrils. Dominant flightlessness is reduced by Ts(YSt;3Lt)A158. Heteroallelic combinations of l(3)Lak are embryonic lethal. Unhatched eggs contain fully formed, phenotypically normal larvae that move very slowly due to muscle dysfunction. Newly hatched larvae and those removed from the vitelline membrane moved slowly but within a few hours died with a 'collapsed' phenotype, possibly due to the inability to feed. Larval muscles display only areas of thick and thin filament interdigitation, filament orientations are often disorganised and Z-discs are irregular in appearance. Heteroallelic lethality can be rescued by Ts(YSt;3Lt)A158, but not by Ts(YSt;3Lt)D8.