Low fertility is a significant problem in animal production that can result in significant economic losses. Male fertility is particularly important in artificial insemination since each male must fertilize a large number of females. One ram of the Rasa Aragonesa sheep breed (M) showed lower fertility (ewes that lambed/ewes inseminated: median: 0.57, interquartile range: 0.35) and fecundity (lambs born/ewes inseminated: 0.96 ± 0.513) values than another (O) that was considered to represent a normal male (fertility: median: 0.73, interquartile range: 0.33; fecundity: 1.22 ± 0.523). In this exploratory case comparison, semen samples from both males were submitted to a complete study of semen quality; semen from M was shown to be of lower quality than that from O. Using testicular biopsies from M and O, RNA sequencing, differential gene expression analysis, and GO enrichment analysis were performed in order to identify differentially expressed genes that could explain these differences. The KRT2 gene was downregulated in M compared to O, with potential effects on membranes. Since only two single individuals were considered, this transcriptomic finding should be interpreted as hypothesis-generating rather than confirmatory. Therefore, for KRT2 to be considered a likely candidate to explain the observed differences in sperm quality and fertility outcomes, a larger number of animals with either high or low fertility should be considered, and validation using qRT-PCR should be performed.