Eleven of 52 species were relatively abundant in the mesopelagic and upper bathypelagic layers in the Rockall Trough and Porcupine Seabight. The fish belonging to the families Gonostomatidae, Myctophidae and Sternoptychidae were the most abundant. During the daytime most members of these three family groups occurred in the depth of 300-900m. Over this depth range gonostomatids and sternoptychids were almost mutually exclusive while gonostom$grave{a}$tids and myctophids showed much greater overlap, as did myctophids and sternoptychids. During the night the distribution of these three families were relatively well separated, myctophids occurring in the surface 100m, sternoptychids at 200-500m and gonostomatids at 500-900m. Generally vertical pattern in the number of species in both areas was similar through day and night with the highest number occurring at 500-900m. The numerical abundance in the epipelagic layer was greater at night than during the daytime, while it was greater in daytime in the upper mesopelagic layer. There was no great diel change in the lower mesopelagic layer. Most of diel migrators in the upper layers of waters at night appear to be mainly from the upper mesopelagic layer. In some species larger individuals tend to occur in the deeper water than smaller ones, suggesting habital segregation with size.