Explaining the evolutionary maintenance of sex remains a key problem in evolutionary biology (1–3). One potential benefit of sex is that it may allow a more rapid adaptive response when environmental conditions change, by increasing the efficiency with which selection can fix beneficial mutations (4–7). Here I show that sex can increase the rate of adaptation in the facultatively sexual single-celled chlorophyte Chlamydomonas reinhardtii, but that the benefits of sex depend crucially on the size of the population that is adapting: sex has a marked effect in large populations but little effect in small populations. Several mechanisms have been proposed to explain the benefits of sex in a novel environment, including stochastic effects in small populations, clonal interference and epistasis between beneficial alleles. These results indicate that clonal interference is important in this system.