Sexually mature qtc¹ males perform much more courtship in response to each other than do control Canton-S-5 (CS) males; mature qtc¹ males have a male-male courtship index (CI) of 42 +/- 5 (indicating that courtship behaviours are observed on average during 42% of a 10 minute observation period), whereas CS males have a CI of only 4 +/- 1. qtc¹ males' courtship of each other is qualitatively different from that of CS controls. In all pairs of CS males observed, one or both males perform at least one of the "early" behaviours (orientation, tapping and following), but in only half of the pairs observed do one or both males vibrate their wings to produce a courtship song and none of the males lick or attempt copulation. In contrast, in all pairs of qtc¹ males observed, one or both males lick the other male's genitalia and one male attempts to copulate with the other male. qtc¹ males do not show chaining behaviour, and like CS males, all of the qtc¹ males run away from the males that court them (indicating that the response to male courtship is normal in qtc¹ males). qtc¹ males perform high levels of male-male courtship in response to CS males, while CS males perform little or no courtship in response to qtc¹ males. In qtc¹/CS pairs in which both males perform at least some courtship, the qtc¹ male initiates courtship first. qtc¹ males initiate courtship of CS virgin females quicker than do control CS males. They perform normal levels of courtship towards the females during the observation period: CI is 75 +/- 3 for qtc¹ males paired with CS females compared to 78 +/- 3 for CS males paired with CS females. The mutant males perform "advanced" courtship behaviours towards the females; all show wing vibration and most show licking of the females' genitalia and curling of the abdomen to attempt copulation. The copulation latencies of qtc¹ males are not significantly different from those of CS males. qtc¹ females elicit as much courtship from CS males as do CS females. Immature qtc¹ males elicit the same level of courtship from CS males as do immature CS males. qtc¹ females are as likely to mate with CS males as are CS females, and the mutant females' copulation latencies are normal. Sensory function of qtc¹ flies appears normal (in limited testing of visual and olfactory physiology by electroretinogram and electroantennogram recordings and tests of adult and larval olfactory behaviour). No general behavioural defects in activities such as walking or grooming are seen.