A heterologous in vitro fertilization (IVF) test using calf oocytes with zona pellucida was employed to assess the fertility of thawed ram sperm samples. Six males with significant differences in fertility (P=0.003) were used. The males were classified as having high fertility (>or=42%) and low fertility (<or=41%). Male fertility was not influenced by number of inseminated ewes (P=0.584), insemination technician (P=0.156), insemination date (P=0.323) or farm (P=0.207). Thawed sperm samples were employed to assess several sperm parameters for each male: motility, acrosomal integrity, viability, membrane stability, membrane phospholipid disorder, mitochondrial membrane potential and chromatin stability. These samples were used to carry out a heterologous in vitro fertilization. In vitro-matured calf oocytes (n=716) were inseminated with thawed ram semen and in vitro cultured for 40 h. Overall, at thawing, variability among males respect to sperm quality was high. Despite this variability, there were not differences (P<0.05) between fertility groups. Yield of hybrid embryos ranged from 31 to 59% between males. There were not differences between males (P=0.340). However, there were differences between fertility groups (high fertility: 55%; low fertility: 39%; P=0.020). Multiple regression analysis showed that the heterologous in vitro fertility was the only predictive parameter for in vivo male fertility. Correlation between both parameters was fair (r(2)=0.760; P=0.025). These results indicate that heterologous in vitro fertilization tests can be useful to predict the fertility of ram spermatozoa using calf oocytes with intact-zona pellucida.