Sperm competition occurs when sperm from different males compete for fertilization. The aim of this in vitro fertilization study was to compare the competitive success of five males using heterosperm with an equal number of sperm from each male and four different sperm/egg ratios (5,000, 10,000, 20,000 and 100,000 spermatozoa per egg) in order to better understand variations of competitive fertilization. The roles of sperm motility and velocity were studied. Fertilization and hatching rates of the 5 males studied, with 100,000 spermatozoa per egg, were between 23.7-94.8% and 23.7-92.2%, respectively. Sperm velocity and percentage of motile sperm ranged from 85.0 to 137.6 μm s-1 and 2.0 to 93.5% at 15 sec post sperm activation, respectively. The contribution of individual males to the progeny when using heterosperm was explored by DNA fingerprinting and was very diverse. Males with very low level of sperm motility (M3 4.45%, M4 1.95%) were represented by low contribution in all groups of progenies, but significant differences in the contribution of males were also found among individuals with a similar percentage of motile spermatozoa. The overall number of spermatozoa per egg strongly influenced the fertilization/hatching rate when using heterosperm, but had no impact on the relative number of progenies sired by each of the 5 competing males. Several models were tested in order to attempt to explain the contribution of individual males to the progeny.