Abstract.The utilization of empty holes of endolithic bivalves(Gttsirocluwnu dubuibyBknnius chdmcitinus, andLitliophaga lithophagaby all other species) as shelters by egg‐guarding males of nine species of AdriaticBlenniidaeis compared. A wide spectrum of entrance sizes to these “standardized” shelters is used with the most frequent utilization ranging between 1(H) and 275 mm:(11.2–18.8mm mean entrance diameters). As a trend within and between species, larger fish utilize holes with disproportionately wider entrances. There arc significant differences between species in the means of “fit” of the heads within the entrances: the heads ofB. dulmaibuts, B. canaeve, B. adrialicus, andB. zvonimiri“fit” tightly into their entrances whereas the two large species,B.puvoandB. tentacularis, which only facultatively use holes of boring mussels for breeding, arc found in holes with “oversized” entrances. In most species mean shelter depth approximates mean body length. Species which tightly “fit” their entrances tend to stay longer when disturbed mechanically or chemically compared to those with only a loose “fit”. The ecological and evolutionary significance of