ABSTRACT: A population of Branchiostomafloridae in Tampa Bay, Florida, USA was sieved from the substratum frequently (often daily) from June 1992 through September 1994. Body lengths were measured for 54264 juvenile and adult lancelets. The breedingseason lasted each year from early May through early September, and newly metamorphosed lancelets settled as juveniles from late May through mid October; during this period of the year, distinct settlements occurred approximately every 1 to 3 wk.Post-settlement growth was followed as changes in modal length on size-frequency histograms. Changes in cohort growth over this period were compared to several different simple and seasonally oscillating growth models. The von Bertalanffy function, insimple and oscillating forms, provided the best estimates of lancelet growth. The lancelets grew in summer (almost 0.5 mm d^-1 in recently settled juveniles), but growth slowed and almost ceased during winter. B. floridae can liveat least 2 yr and can reach a maximum length of 58 mm. The maximal secondary production was 61.53 g m^-2 yr^-1 (ash-free dry weight) and the production to biomass ratio was 11.64. Population densities at the study site ranged fromabout 100 to 1200 lancelets m^-2.