Lancelets of the genus Branchiostoma (amphioxus) are widespread and locally abundant filter-feeding animals in shallow coastal waters of the south-eastern US (up to 5000 ind/m2) and in temperate and tropical seas worldwide (up to 9000 ind/m2). Lancelets are consumed by bottom-dwelling fish and humans. As part of a larger project to aquaculture lancelets, an experiment was conducted to determine the range of diameters of suspended particles filtered and ingested by the Florida lancelet, Branchiostoma floridae (Chordata: Cephalochordata). After a period of starvation, animals were exposed to a suspension of tracer particles of seven different diameters (range 90–0.062 μm) and the protein, ferritin (0.012 μm) and their faeces were examined subsequently for the presence or absence of tracer. Particles ranging from 90–0.062 μm, but not ferritin, were filtered and ingested. Many of the 90 μm diameter particles, however, were excluded from entering the body by the oral cirri. Under experimental conditions, B. floridae filters and ingests particles in the range of ∼100–0.062 μm (microplankton to colloidal particles). This result suggests that the lancelet diet, like that of appendicularians, includes microbial as well as phytoplankton production.