Growth parameters of Mesophyllum alternans and Lithophyllum frondosum were studied in the northwestern Mediterranean. Growth and shrinkage rates, in terms of area gained or lost per month, were measured for both crustose corallines over a 2 year period by photographic and computer-assisted image analysis. No seasonal pattern in growth or shrinkage was found for either species. M. alternans grew and shrank at significantly higher rates than L. frondosum. Growth rates for both species were over an order of magnitude lower than those previously reported for other Mediterranean and tropical species, and were similar to reports for crustose corallines in Arctic and temperate waters. Almost 50% of monitored M. alternans thalli underwent at least one fission or fusion event over the 2 year monitoring period, while in L. frondosum these events were rarely observed. Interspecific differences in rates of growth, shrinkage, fission and fusion appear to depend on differences in thallus thickness (M. alternans is thin and L. frondosum is thick). These differences were interpreted as distinct biological strategies of persistence and the occupation of primary substratum.