Chaetognaths present at a 24-h station located off the Ebro River delta in the western Mediterranean basin were studied. No vertical migratory behaviour related to the light cycle was observed in the total chaetognaths as a whole, but the behaviour of the individual species present in the area, Sagitta enflata, S. setosa and S. minima, did present differences. S. enflata migrated through the water column during the day, whereas S. setosa and S. minima did not exhibit any appreciable differences in vertical distribution. Considering the sexual maturity stages of these species, adult S. enflata and S. setosa carried out distinct vertical migrations, while none of the maturity stages of S. minima displayed migratory behaviour, and this species remained below the thermocline both in the daytime and at night. It is interested to note in the area the presence of Pterosagitta draco, because this species is scarce in the Mediterranean.