A study has been made of the distribution of the lancelet Branchiostoma nigeriense in the neighbourhood of Lagos. From observations on the development of fertilized ova in the laboratory, an estimate of the duration of the embryonic period in this species has been made. Systematic collections of both larvae and adults were made during the years 1953 to 1955, and from these the duration of the life cycle and the rate of growth of both the larva and the adult have been assessed. It has been established that lancelets in the brackish lagoons at Lagos are derived annually from marine populations outside the harbour. The larvae enter brackish water when the salinity is not lower than 13 parts per thousand in the autumn, metamorphose at the end of December and give rise to adults which colonize the sand deposits of the lagoons. The adults reach maturity and spawn in the spring, but die shortly afterward when the salinity of the water falls below the threshold value for survival. The marine populations have an annual cycle, spawning from August to October or November each year.