To assess the ways in which different selection pressures on male and female spinyhead blennies (Acanthemblemaria spinosa) influence their use of space, we measured a number of features of their shelter sites. Both sexes occurred primarily on dead surfaces of elkhorn coral (Acropora palmata), but significantly more males than females were found on live coral surfaces. Males occurred twice as frequently as females in the vacated constructed tubes of serpulid worms or vermetid molluscs, which provide greater surface for egg deposition than the shorter cavities excavated by other organisms. There was no difference between the sexes in shelter height above the reef surface. Fish size was positively correlated with shelter height above the reef surface for both sexes. Number of eggs being guarded by males was positively correlated with both fish size and shelter height above the reef surface; in concert they account for 44% of the variance in egg number. To assess the effects of competition, we removed spinyheads from corals and documented recolonization along with shelter site parameters. Corals originally harboring larger fish were recolonized at a higher rate, and in all cases the mean size of recolonists was smaller than originals; the original correlation of fish size with height above the reef surface collapsed for recolonists. These observations are consistent with others that indicate that spinyhead blennies compete for quality shelters.