Abstract We have analysed the effect of 288 generations of mutation accumulation (MA) on chromosome II competitive fitness in 21 full-sib lines of Drosophila melanogaster and in a large control population, all derived from the same isogenic base. The rate of mean log-fitness decline and that of increase of the between-line variance were consistent with a low rate (λ ≈ 0.03 per gamete and generation), and moderate average fitness effect [E(s) ≈ 0.1] of deleterious mutation. Subsequently, crosses were made between pairs of MA lines, and these were maintained with effective size on the order of a few tens. In these crosses, MA recombinant chromosomes quickly recovered to about the average fitness level of control chromosomes. Thus, deleterious mutations responsible for the fitness decline were efficiently selected against in relatively small populations, confirming that their effects were larger than a few percent.