We describe Elasmopus warra (Crustacea, Amphipoda) from Pearl Beach in the vicinity of Sydney, Australia. We also investigate the patterns of spatial variation of Elasmopus in coralline algal turf on rocky intertidal shores in this area and examine experimentally the role that two architectural characteristics of coralline turf, the length and density of fronds, have in determining these patterns of abundance. Initial sampling showed that E. warra was more abundant in coralline turf in low- than in mid-shore areas, and often its abundance varied between patches of turf low on the shore. The results of experiments showed that the length of fronds had a much greater effect on the abundance of E. warra than the density of fronds. In low-shore areas, increasing the length of fronds negatively affected the abundance of E. warra. Such a relationship has rarely been reported in the literature, and possible causes for this relationship are discussed. This study showed that the nature of effects of marine plant architecture on the abundance of small crustaceans depend on local conditions and the architectural characteristics being investigated.