The aim of the present study was to characterise European eel spermatozoa morphometrically, as a basis for future studies on the morphological effects of methods for sperm cryopreservation and sperm quality. This characterisation was carried out measuring several spermatozoa morphology parameters (head length, width, area and perimeter) by scanning electron microscopy (SEM), in comparison with measurements developed in European eel spermatozoa with computer-assisted morphology analysis (ASMA). Spermatozoa head morphology showed differences in width (1.15+/-0.01 microm versus 1.12+/-0.01 microm), perimeter (14.68+/-0.13 microm versus 13.72+/-0.19 microm) and area (5.36+/-0.06 microm2 versus 1.12+/-0.01 microm2) for ASMA and SEM, respectively. When head length was evaluated, significant differences were found, being higher for SEM methodology (5.09+/-0.04 microm versus 4.29+/-0.03 microm). The curved and elongated spermatozoa head in eels means a problem for the ASMA system (Sperm Class Analyser), Morfo Version 1.1, Imagesp, Barcelona, Spain), causing an error in the length measurements. However, similar results were obtained by both techniques when spermatozoa head length was considered as the greater length between two points within the object (4.29+/-0.03 microm versus 4.31+/-0.04 microm for ASMA and SEM, respectively). In conclusion, this is one of the first applications of ASMA in fish and the first in this species, and confirms this system as a useful tool with wide applications in future fish spermatozoa studies. Width, perimeter and area could be used as parameters for the spermatozoa morphology evaluation, whereas the length requires a new programming of the Imagesp software.