Cl^- channel protein 1 (ClC-1) may be important for excitability and contractility in skeletal muscle, but ClC-1 abundance has not been examined in human muscle. The aim of the present study was to examine ClC-1 abundance in human skeletal muscle, including fibre type specific differences and the effect of exercise training. A commercially available antibody was tested with positive and negative control tissue, and it recognised specifically ClC-1 in the range from 100-150 kDa. Abundance of ClC-1 was 38% higher (P < 0.01) in fast twitch Type IIa muscle fibres than in slow twitch Type I. Muscle ClC-1 abundance did not change with four weeks of training consisting of 30 min cycling at 85% of maximal heart rate (HR_max) and 3×30-s all out sprints or during a 7-week training period with 10-12×30 s uphill cycling and 4-5×~4 min cycling at 90-95% of HR_max. ClC-1 abundance correlated negatively (P < 0.01) with maximal oxygen consumption (r = -0.552) and incremental exercise performance (r = -0.546). In addition, trained cyclists had lower (P < 0.01) ClC-1 abundance than lesser trained individuals. The present observations indicate that a low abundance of muscle ClC-1 may be beneficial for exercise performance, but the role of abundance and regulation of ClC-1 in skeletal muscle of humans with respect to exercise performance and trainability need to be elucidated.