In this paper we present an experimental test of the deterministic mutation hypothesis on the evolution of sex. We studied the direct effect (i.e. before selection) of sex on offspring fitness of two strains of the unicellular alga Chlamydomonas moewusii, that had been kept in the laboratory for over 60 years. The logistic parameters r and K of each genotype, estimated in batch culture, were used as a measure of fitness. Strains were treated with UV to cause additional deleterious mutations. By comparing mean log fitness of parents and offspring in relation to the fitness difference of the parents, we tested whether and how deleterious mutations interact. No significant recombinational load was found in the offspring of the untreated strains. However, a significant negative effect of sex on log r and log K was found after crossing UV treated strains. We argue that this negative effect of sex on fitness suggests synergistic interaction, at least between the UV-induced and the naturally accumulated deleterious mutations. The latter result therefore supports the deterministic mutation hypothesis.