An analysis of shell microstructure of Anadara granosa and Perna viridis experimentally cultured in coastal waters of Thailand revealed that the formation of growth lines was closely related to tidal periodicity. It is concluded thatgrowth lines are formed when the bivalves react with shell closure to reduced water flow, the degree of sensitivity being species-specific. The application of the methodology to tropical bivalves seems, however, not recommendable, given the rather demanding preparative work and the large degree of uncertainty involved in the interpretation ofobserved growth lines. Fitting of growth data from the culture experiments to the von Bertalanffy growth function showed large variations of the single growth parameters, but little variance in the resulting growth index, Phi ', supporting the perception of Phi ' being a species-specific parameter. It was shown that Phi ' is usually narrowly distributed around a species-specific mean. Use of Phi ' for growth comparison between different genera is generally limited because of the different shell shape, which strongly influences Phi '. However, mytilids with their relative uniform shell shape displayed a linear increase of Phi ' with environmental water temperature, suggesting thatbivalves tend to grow larger in warm waters.