ABSTRACT: Effects of salinity, temperature and food concentration on early development of the polychaete Hydroides elegans (Haswell) were examined in 4 laboratory experiments. Three 2-factor experiments tested the effects of salinity (15 to 35o/oo) andtemperature (15 to 30°C ) on the survival and duration of development of H. elegans from newly-released oocyte to 2-cell, 2-cell to blastula, and blastula to trochophore stages respectively. A fourth 3-factor experiment tested the effects ofsalinity (15 to 35o/oo), temperature (15 to 30°C ), and concentration of the single-cell alga Isochrysis galbana (0 to 10⁶ cells ml^-1) on survival, settlement, and duration of development from trochophore to newly-settledjuvenile. Within the experimental range, temperature had no effect on survivorship, but low temperature led to longer duration of development. Low salinity reduced survivorship and settlement, and lengthened the duration of development. Low foodconcentration reduced survivorship and settlement, and lengthened the duration of development from trochophore to newly-settled juvenile. At concentrations <=10³ cells ml^-1, >35% larvae survived through the 10 d experiment but losttheir ability to become competent. Percentages of trochophores reaching settlement were similar at 10⁴, 10⁵, and 10⁶ cells ml^-1. Duration of development was shortest at concentrations of 10⁵ cellsml^-1, while trochophores at 10⁴ and 10⁶ cells ml^-1 had similar but longer durations of development. Our data suggest that in Hong Kong waters, the decrease in salinity during the summer seems to override thebenefits of high temperature and to be responsible for the decline in H. elegans settlement. The increase in phytoplankton concentration from early spring to early summer may contribute to the formation of settlement peaks. Temperature, however,does not seem to be a limiting factor for early development and settlement of H. elegans.