There is no information about the characteristics of early cleavage in the Patagonian blennie (Eleginops maclovinus), which can be used as a diagnostic tool for embryo quality. The purpose of this investigation, therefore, was to characterize the first blastomeres of E. maclovinus morphologically. Of a 'pool' of incubated eggs at 10.7 +/- 0.5 degrees C, 100 microphotographs of blastodiscs were extracted at different incubation periods from 0.25 to 5 h after fertilization and analyzed. Blastodiscs taken at 3.5, 4.0 and 5.0 h were characterized and classified into symmetric or asymmetric groups according to their morphology. The proportions of length (L) and width (W) of each blastomere were determined to establish its symmetry. Additionally, 20 microphotographs of blastodiscs of normal appearance were analyzed morphologically (control blastodisc: CB) and compared other blastodiscs (4.0 and 5.0 h). The results showed that before fertilization oocytes presented a somehow turgid aspect (maximum average diameter of 987 +/- 41 mu m) and after fertilization and hydration, their diameter increased to 1001.5 +/- 11 mu m (but not statistically significant) and presented a spherical shape. First cleavage ends after 3.5 h of development, forming two blastomeres 467 +/- 45 mu m length (L) and 328 +/- 21 mu m width (W) with a L/W ratio of 1.43 +/- 0.19. The second cleavage ends after development at 4.5 h forming four blastomeres 238 +/- 65 mu m length and 227 +/- 65 mu m width with a ratio L/W of 1.06 +/- 0.09. Five categories were identified during the blastomere characterization: 70% normal or symmetric; 8% with odd numbers of blastomeres; 6% unequal; 6% 'pie shaped' and 10% amorphous.