This study was designed to explore the relationships between morphology, motility, osmolality, and fertilization success of rainbow trout (Oncorhynchus mykiss) spermatozoa. Sperm morphology was analyzed using automated software on stained (Diff-Quick-stain) smears examined under the light microscope. Spermatozoa with longer flagellae were observed to show higher straight-line velocities (VSL) and a more linear movement than those with shorter flagellae. Fertilization success was inversely related to the variables (i) duration of sperm movement, (ii) linearity and (iii) density. Strong relationships were also observed between head and tail lengths and between VSL and fertilization capacity although significance was not reached; the former relationship being positively and the latter inversely related. When outliers were removed, positive correlation was detected between the duration of sperm movement and density and between mean curvilinear velocity (VCL) and the fertilization success. According to these findings, we propose that longer spermatozoa could reach the micropyle of eggs before shorter spermatozoa, but this would only hold for sperm expelled close to the micropyle. If expelled at a distance, then the shorter sperm undergoing more curvilinear movements would find it easier to circulate an egg and fertilize it.