This study documents the fauna in the size range 0.1–5.0 mm found on the encrusting brown algaPseudolithodermasp., common on patches of the honeycomb barnacleChamaesipho columnaon mid‐intertidal shores in northern New Zealand. Collections of fauna were made in four different habitats associated with this alga and in the presence or absence of underlying barnacles: (1) maturePseudolithodermacrust on barnacles, (2) maturePseudolithodermacrust occurring on bare rock, (3)Pseudolithodermaflaking off rock, and (4) barnacles withoutPseudolithoderma. The invertebrate species composition differed from assemblages recorded for other, more structurally complex algae, such as coralline turfs. More species were found onPseudolithodermathan on bare barnacles. There were no significant differences in the number of species between different habitats containingPseudolithoderma;however, flakingPseudolithodermahad fewer individuals than otherPseudolithodermahabitats. We hypothesise that less structurally complex habitats, such as encrusting algae, are more suitable to fauna with protective shells or tubes; and speculate that the large numbers of such fauna in this assemblage influence the ecology ofPseudolithoderma.