Abstract This thesis deals with factors affecting mate choice as well as with genetic variation in male courtship song in Drosophila montana. Males, which produced song with a high carrier frequency, were found to court females, and also to succeed in their courtship more often than the males producing low frequency song. Male mating success correlated with the carrier frequency of his song recorded after, but not before, an "artificial winter", which suggests that a sexually selected male trait is sensitive to environmental factors. A high carrier frequency of male courtship song correlated positively with the survival rate of the male's progeny from egg to adulthood (indirect benefit for the female), but not with the fecundity of his mating partner (no direct benefit for the female). The heritabilities and the amount of additive and residual variation in male courtship song characters were measured in two populations using father-son regression and sib analysis. The songs of the males from one of these populations were analysed for a second time after the cold treatment. Most heritability values were insignificant, largely due to high residual variation. During the cold treatment, the additive variation increased and the residual variation decreased in almost all song traits. Increased variation in sexually selected traits may help the females to exercise selection between the males during the mating season of the flies in the wild in spring. This, and the fact that male song gives the female information about the male's condition/genetic quality suggests that in this species the evolution of female preferences for male song characters could have evolved through condition-dependent viability selection as postulated by "good genes" models. Variation and inbreeding depression/heterosis were studied in traits associated with fly reproduction using inbred D. montana strains. Songs, hydrocarbons and some behavioural traits of the flies varied significantly between strains. The strain of both sexes affected female egg-laying, and the female strain, also, the survival rate of the flies' progeny, in different intra- and interspecific combinations. Heterosis was found in the mating propensity of the flies and in the carrier frequency of the male song. Diallel analysis revealed unidirectional dominance towards higher carrier frequency. This direction is the same as the direction of sexual selection exercised by the females of this species suggesting that sexual selection could be a driving force in evolution of this song trait.