The coexistence of three taxa of Codium (Chlorophyta: Codiaceae) was examined on wave-swept shores of western Ireland: the introduced macroalgae Codium fragile ssp. atlanticum and ssp. tomentosoides and the native Codium tomentosum. In spring 1999 and 2000, four sites were monitored to evaluate whether past predictions regarding temporal changes and species replacement occurred. Introduced and native species of dichotomously branching Codium occurred at four tidal levels: in tidepools in (i) the upper barnacle zone; (ii) the mid barnacle/fucoid zone; (iii) the mussel zone, and (iv) on emergent substrata in the red algal turf/kelp zone. Codium fragile ssp. atlanticum and ssp. tomentosoides grew together in 11·9% of surveyed pools containing Codium. Compared to 1971 censuses, C. fragile ssp. atlanticum had increased in relative abundance at all tidal levels, and the dominance of C. fragile ssp. tomentosoides had declined. The native C. tomentosum constituted the same percentage of the population as it did in 1971. Pool availability and primary rock surfaces were not limiting at most sites, indicating that inter-subspecific and inter-specific competition could not account for temporal changes in Codium distribution on Irish shores. Ecological constraints other than intra-generic competition may have contributed to the purported long-term changes in Codium abundance.