Egg production and viability in the copepod Temora stylifera (collected in the Bay of Naples, Italy in 1992) were strongly dependent on food type. A flagellate (Isochrysis galbana) diet induced the production of good quality eggs that developed to hatching. By contrast, two diatoms ( Chaetoceros curvisetum, PhaeodactyIum tricornutum) resulted in poor egg quality, with hatching success as low as 20% of total egg production. With the third diatom tested, Skeletonema costatum, females produced eggs for only 3 to 4 d, after which time they either became sterile or died. These results are discussed in relation to previous findings regarding the impact of the dinoflagellate Prorocentrum minimum and the diatom ThaIassiosira rotula on the hatching success of T. stylifera eggs. Low egg viability was possibly not due to an absence of remating or a deficiency of some specific essential nutrient required for egg development but to the presence of inhibitory compounds blocking cell division during early copepod embryogenesis. This questions the traditional view that diatoms are an important food item regulating copepod secondary production.