We investigated factors that may determine mate guarding tactics in male sand lizards. In a sample of lizards from a museum collection, larger males had larger testis, but in laboratory experiments and in a natural population larger males did not sire more offspring. Males with long inter-copulatory intervals were more successful in sperm competition than males with short inter-copulatory intervals. In the wild, the operational sex ratio (OSR, No of receptive females/No of sexually active males) declined throughout the mating season. Mean duration of mate guardings was unaffected by OSR, time to ovulation, female age and mass, and clutch size. Larger males guarded females longer and were more likely to mate guard a female of similar age. Larger males had more partners but there was no correlation between male size or guarding time and the proportion of young that males sired in clutches from females mated with several partners. Males with more partners were more successful at siring offspring in clutches from females that mated with more than one partner. We suggest that fitter males are better at both mate acquisition and have more competitive sperm.