The purposes of this study were to establish relationships between selected underwater kinematics and the starting and turning performances and to quantify kinematic differences between these segments in sprint butterfly swimming. Fourteen male swimmers performed 50 m maximal butterfly swimming in a short course pre-calibrated pool. The entire race was filmed by a multi-camera system, which quantified the forward head displacement and velocity (vx_head) throughout the race with a sampling frequency of 50 Hz. The time taken between 0-15 m (T_0-15) and 25-35 m (T_25-35) as well as 16 kinematic variables were acquired from the data provided by the system and manual video processing for further analysis. The mean underwater velocity (UW-vx_mean) was related to both T_0-15 and T_25-35 (r = -0.70 and -0.95, respectively; p < 0.01). UW-vx_mean was positively correlated with vx_head during the first kick (r = 0.84, p < 0.001) in the start segment and with vx_head during the last kick in the turn segment (r = 0.68, p < 0.01), but other kinematic variables such as kick frequency, body angle, deceleration during kicks (Dec_kick), and glide time were not related to UW-vx_mean. Swimmers had larger vx_head at the beginning of the segment and during the first kick in the start than in turn segment (p < 0.001). However, vx_head during the last kick was similar due to the larger Dec_kick (p < 0.05) in the start than in turn segment. The underwater time was similar between the segments despite a longer underwater distance (p < 0.01) and a larger kick count and frequency (p < 0.01) in the start than turn segment. In conclusion, UW-vx_mean is an important factor for start and turn performances, but swimmers select individual kinematic strategies to achieve a large UW-vx_mean. Results also highlighted the importance of the different parts within the underwater segment in each segment.

• In 50 m butterfly swimming, few kinematic variables were linearly related to start and turn performances due to a large inter-individual variability.

• The mean forward velocity during the underwater phase was largely correlated with start and turn time. The mean velocity was related to the first kick velocity in the start segment and to the last kick velocity in the turn segment.

• In 50 m butterfly swimming, the initial forward velocity (the velocity at the feet-entry and the push-off) was larger in the start than in the turn segment, but the forward velocity at the last kick before the breakout was not different between the segments.

• In 50 m butterfly swimming, the underwater distance is longer in the start than in the turn segment, but the underwater time is similar between the segments. Despite the similarity in the underwater time, swimmers perform a larger number of kicks with higher kick frequency in the start than the turn segment.