A fish propelling itself by means of the pectoral fins, was filmed with a high speed cine camera. For each film frame the movements of the pectoral fins were analysed and the body velocity calculated. The fin beat cycle can be divided into three phases. The ef fects of each phase on the body velocity are variable, but downstrokes generally cause deceleration and upstrokes acceleration. The intermediate phase sometimes showed acceleration possibly caused by a jet of water squirted out from the space between fin and body wall. The discrepancy between the observed fin shapes and the simplified representation of the fin as a flat plate is discussed. Estimates of drag and swimming force are made.