CHATARD, J.-C, X. SENEGAS, M. SELLES, P. DREANOT, and A. GEYSSANT. Wet suit effect: a comparison between competitive swimmers and triathletes. Med. Sci. Sports Exerc., Vol. 27, No. 4, pp. 580–586, 1995. The purpose of the present investigation was to examine the wet suit effect on 8 swimmers and 8 triathletes. For swimmers, the performances of a 400-m swim with and without wet suit were not statistically different (4 min 12.5 ± 8 s vs 4 min 13.9 ± 4 s) while for triathletes the swim times were reduced by 19 s (4 min 45.8 ± 34 s vs 5 min 04.7 ± 30 s, P > 0.01). For swimmers, VO2max, and blood lactate measured with the wet suit were lower than without (P > 0.01), while for triathletes stroke rate was significantly higher with the wet suit (P > 0.01). For the whole group, the individual differences of performance were related to the blood lactale differences (r = −0.68; P > 0.01) and to the hydrostatic lift (r = 0.63; P > 0.01). For swimmers, the energy cost of swimming and the gliding ability were not statistically different with or without wet suit, while for triathletes they were significantly lower and decreased with velocity. It is concluded that the wet suit effect improves performance more in inefficient swimmers with low buoyancy, swimming at low speeds.