The zebrafish Danio rerio exhibits substantial morphological variability in the sizes and shapes of the body and the caudal fin. The present study describes swimming performance, swimming behaviour and routine locomotor activity patterns in three of the major morphotypes: wild-type, long-finned and no-tail. Wild-type and long-finned differ in total length (TL), fork length (FL), caudal fin length (CFL) and caudal fin height (CFH). No-tail has no caudal fin and is significantly smaller in standard length (SL) than the other types. Critical swimming speeds (U(crit)) were measured at 28 degrees C in a modified Brett-type water tunnel. U(crit) of wild-type fish was 56.0+/-4.8 cm s(-1) or 15.5 SL s(-)(1) (mean +/- s.d., N=21), significantly faster than the U(crit) of long-finned fish (43.7+/-6.8 cm s(-1) or 12.5 SL s(-1), N=17); both were significantly faster than the U(crit) of no-tail fish (19. 8+/-4.7 cm s(-1) or 6.9 SL s(-1), N=15). When forced to swim in the water tunnel, zebrafish tended to turn and swim downstream for short periods at slow water velocities. Turning frequencies (turns per minute, f(T)) at the slowest velocity (4 cm s(-1)) were 10. 1+/-6.5 min(-)(1) (N=63) and 8.6+/-4.7 min(-1) (N=51) for wild-type and long-finned, respectively, significantly different from that of the no-tail fish, 4.7+/-2.8 min(-1) (N=45). These frequencies decreased below 1 min(-1) at 56%, 64% and 61% of U(crit) in wild-type, long-finned and no-tail fish, respectively. Activity levels of wild-type fish were generally significantly higher than those of long-finned fish, and the levels of both were significantly higher than those of no-tail fish. The pattern of differences in relative activity levels between types was similar to that for U(crit). The results show that the wild-type fish, on a size-scaled basis, is one of the fastest-swimming fishes ever measured, reaching the maximum predicted theoretical sustained swimming speed. U(crit) of long-finned fish was 22% lower than that of wild-type fish, and U(crit) of no-tail fish was 65% lower. Similar differences were found in turning frequencies and routine activity level.