The population structure of Sagitta serosa from the south-western part of the Black Sea was examined between May 1994 and December 1996. In addition, the vertical distribution and diel vertical migration of S.setosa were studied for juveniles (less than or equal to 5 mm) and adults (>5 mm) in April 1995, September 1995 and June 1996. The population structure studies showed that a new generation was introduced in September. Breeding was most intensive from June to November, indicated by the frequency of small-sized individuals. Diel vertical migration from the depth of the oxygen minimum zone to the surface was observed for adult S.setosa but not for juveniles. Juveniles were generally distributed above the seasonal thermocline. It is evident that adults are able to tolerate a wide range of oxygen concentrations and temperature.