The objective of this study was to examine sperm penetration through cervical mucus and associated physicochemical properties of cervical mucus from Belclare and Suffolk ewes - two breeds with divergent pregnancy rate following cervical AI using frozen-thawed semen. In Experiment 1, sperm penetration through cervical mucus was assessed in 15 Belclare and 15 Suffolk ewes at 30, 48 and 57h post sponge removal. In Experiment 2, rheological properties of mucus from 17 Belclare and 19 Suffolk ewes at 48 and 57 h post sponge removal were determined. In Experiment 3, 20 Belclare and 20 Suffolk ewes were used to assess mucus ferning and pH collected at 42, 48, 57 and 65 h post sponge removal. In Experiment 1, a higher number of sperm penetrated cervical mucus from Belclare ewes at 48 h, reflected by a breed by time interaction (P = 0.05). In Experiment 2, mucus from Suffolk ewes tended to have higher elastic and complex moduli than that from Belclare ewes (P = 0.06) regardless of time of collection. There was no effect of ewe breed on the viscous modulus. In Experiment 3, there was a significant effect of time post sponge removal on ferning (P < 0.01), but there was no effect of breed. There was no effect of time or breed on mucus pH. It is concluded that breed differences in the rheological properties of cervical mucus affect the ability of sperm to swim through cervical mucus and this may explain breed differences in fertility observed after cervical AI using frozen-thawed semen. (C) 2011 Elsevier B.V. All rights reserved.