ABSTRACT: Although recent studies have found that some invertebrate larvae can meet part of their metabolic needs through bacterivory, it is unclear to what extent bacterivory can compensate for reduced phytoplankton abundance. The present study determined whetherlarvae of a polychaete (Hydroides elegans)and a barnacle (Balanus amphitrite)can survive, grow, and develop to competence solely on a diet of bacteria. In laboratory experiments, H.elegans larvae provided withbacteria as their sole particulate food source completed larval development, attached, and metamorphosed into healthy early juveniles. In addition, several of these juveniles, when provided with phytoplankton after metamorphosis, developed to maturity andspawned viable offspring. Bacterial abundances measured in Port Shelter, a bay in Hong Kong waters where organisms for the present study were collected, would not sustain maximum development rates of H.elegans larvae. However, bacterialabundances were sufficiently high and consistent over time to constitute a reliable food source. Consequently, starvation may not be a direct cause of larval mortality in this species. On the other hand, B.amphitrite larvae provided withbacteria did not grow, and death occurred at the same age as in starved larvae, suggesting they cannot use bacteria as a supplementary food source. Available data suggests that most barnacle recruitment in Hong Kong waters occurs when phytoplanktonabundance reaches peak levels. Recruitment of H.elegans, however, occurs throughout the year, suggesting that spawning and successful larval development may be independent of phytoplankton availability, and that larvae largely rely onalternate food sources such as bacteria.