Estimations have been made of the salinity tolerance of the larva and adult of the lancelet Branchiostoma nigeriense and the effects of fluctuating salinities on its ciliary feeding mechanism. The results obtained have been correlated with the times of occurrence and the rates of growth of these lancelets in the Lagos area. The reactions of larval and adult lancelets to changes in temperature have been observed and the range of temperature within which these animals can survive has been determined. The behaviour of larval lancelets in salinity gradients has been studied. It is shown that the reactions of the larva to salinity and temperature restrict the animal to waters of high salinity and temperature. The distribution of adult lancelets in the Lagos area has been shown to coincide with that of sand deposits containing not more than 25% of fine grains passing a 90-mesh to the inch sieve or 1<latex>$\cdot $</latex>5% of silt. Experiments have been performed confirming the preference of lancelets for this type of sand and indicating the factors which lead to its selection by the animal through a process of trial and error. The behaviour of B. nigeriense is discussed. The distribution of this lancelet is shown to be a result of its reactions to environment.