Social loafing is the tendency of individuals to work less hard collectively than individually. The present study examined the joint influence of achievement motivation and expected coworker effort on collective task performance. Participants (N= 107) who qualified and were available after pretesting on an achievement motivation scale were randomly assigned to a work condition and coworker effort condition. Dyads were asked to generate as many uses for a knife as possible within a 12‐min time period. Participants low in achievement motivation engaged in social loafing, but only when expected coworker effort was high, whereas participants high in achievement motivation did not engage in social loafing, regardless of expected coworker effort. The implication of achievement motivation for collective task performance settings is discussed.