Purpose. Recent studies have suggested that the age of peak freestyle swimming speed is reached earlier in life in women than in men. However, no study has investigated the age of peak swimming speed in other swimming styles such as butterfly. The aims of the present study were to investigate the age of peak swimming speed in elite male and female butterfly and freestyle swimmers at the national level (Switzerland) and the sex differences in both the age of peak swimming speed and swimming speed for both swimming styles. Methods. Results of the elite Swiss swimmers between 2006 and 2010 were analysed using one-way analysis of variance. Results. In butterfly, women achieved peak swimming speed at 20-21 years in the 50 m, 100 m and 200 m, whereas men reached their fastest swimming speed in the 50 m at 20-21 years and in both the 100 m and 200 m at 18-19 years. In freestyle, women achieved peak swimming speed at 20-21 years for all distances. Men were the fastest at 22-23 years for both the 100 m and 200 m and at 26-27 years for the 50 m. In the butterfly, the sex difference in swimming speed was highest in the 50 m and lowest in the 200 m (14.1% ± 0.2 in the 50 m, 12.6% ± 1.0 in the 100 m and 8.7% ± 1.8 in the 200 m). Additionally, the sex difference in freestyle swimming speed was highest in the 50 m and lowest in the 200 m (16.2% ± 0.5 in the 50 m, 15.9% ± 0.4 in 100 m and 14.9% ± 1.0 in 200 m). Conclusions. These findings suggest that peak swimming speed was achieved earlier in life in men compared with women for the 100 m and 200 m butterfly distances but not in the 50 m butterfly. In freestyle, peak swimming speed was achieved at younger ages in women compared with men. The sex difference in peak swimming speed was lower in the butterfly than in freestyle.