Two trials in 1991 and another two in 1992 were carried out on the broodstock conditioning of Manila clams, Tapes philippinarum. The main objective was to manipulate the lipid and polyunsaturated fatty acid content of Manila clam eggs by maintaining broodstock in different dietary regimes. The second objective was to assess dried algae as alternative diets for conditioning broodstock. Clams were brought into the laboratory from the natural environment early in the year, before gametogenesis had started. Supplements of cultured live (Dunaliella tertiolecta, Skeletonema costatum, Tetraselmis suecica and Isochrysis galbana) and dried algae (T. suecica) diets were fed to the broodstock, usually in a range of mixed diets, at rations equivalent to 3% or 6% of the initial dry meat weight of the broodstock in dry weight of algae per day. The microalgae differed in their long-chain polyunsaturated fatty acid content (PUFA). Unfed control clams received only the organic material which remained in the sea water after sand filtration. The nutritional value of these diets in relation to gametogenesis, fecundity, quality of eggs, and viability and growth of larvae were assessed. Dry T. suecica was the same food value as live T. suecica but Manila clams produced more eggs if supplements of live algae were added. The requirement for conditioning Manila clams (32 mm shell length) to spawn with live or dry T. suecica+S. costatum was 500 to 700 "day-degrees" (D°). With dry T. suecica on its own or mixed with I. galbana, S. costatum and D. tertiolecta it was 500 to 600 Do (44 mm shell length). In one trial clams spawned in the tanks (equivalent to 462 Do) before the first attempt to spawn them was made. Successful spawning was dependent on the quantity and quality of the algal diet during gametogenesis. With a 6% food ration, clams fed dry T. suecica+S. costatum or dry T. suecica+l. galbana produced the highest number of eggs (an average of 3.2 and 4.5 million eggs per female, respectively). The average fecundity was 83% lower when the diet was reduced to a 3% food ration. The dry meat weight, condition index and fecundity of fed broodstock were significantly higher than for unfed animals. The quantity of lipid in the eggs, usually between 4 and 9 ng egg-1, was similar whatever the broodstock diet. However, levels of the essential polyunsaturated fatty acids 20: 5w3 and 22: 6w3 in the eggs were low if the broodstock diet was deficient in these PUFAs. Even though diet manipulation caused changes in the fatty acid composition of the eggs, growth and survival of Manila clam larvae was not reduced in a hatchery situation.