Populations of Ischnura elegans, Coenagrion puella and Enallagma cyathigerum were studied in order to determine sex ratios, mating frequencies and mating expectancies. As the true imaginal sex ratio is unknown in these Odonata attempts were also made to determine the larval and final instar exuvial sex ratios. The larvae of I. elegans showed a small but nonsignificant excess of males, and males constituted 62 percent of imaginal samples. Estimates of total season's imaginal population sizes tend to suggest that the real sex ratio was near to I:I. Larval samples of C. puella indicated a significant excess of males and males constituted 76 percent of imaginal samples. Samples of final instar exuviae of E. cyathigerum showed that the sex ratio at emergence was exactly unity but male imagines constituted 84 percent of the population samples. Only 5 percent male and I2 percent female I. elegans were seen to mate despite a high persistence at water and that this species has a long copulation time. Sexual maturation is rapid in I. elegans and both sexes mate well into old age. The mating frequencies of male C. puella and E. cyathigerum were low, but females of these species showed very much higher values than for I. elegans. The data suggest that some males of I. elegans and E. cyathigerum are not successful in achieving mating.