Trophodynamics of blooms of the toxic marine cyanobacterium Lyngbya majuscula were investigated to determine dietary specificity in two putative grazers: the opisthobranch molluscs, Stylocheilus striatus and Bursatella leachii. S. striatus is associated with L. majuscula blooms and is known to sequester L. majuscula metabolites. The dietary specificity and toxicodynamics of B. leachii in relation to L. majuscula is less well documented. In this study we found diet history had no significant effect upon dietary selectivity of S. striatus when offered a range of plant species. However, L. majuscula chemotype may alter S. striatus' selectivity for this cyanobacterium. Daily biomass increases between small and large size groups of both species were recorded in no-choice consumption trials using L. majuscula. Both S. striatus and B. leachii preferentially consumed L. majuscula containing lyngbyatoxin-a. Increase in mass over a 10-day period in B. leachii (915%) was significantly greater than S. striatus (150%), yet S. striatus consumed greater quantities of L. majuscula (g day- 1) and thus had a lower conversion efficiency (0.038) than B. leachii (0.081) based on sea hare weight per gram of L. majuscula consumed day- 1. Our findings suggest that growth rates and conversion efficiencies may be influenced by sea hare maximum growth potential, acquisition of secondary metabolites or diet type.