In Ireland, liquid bull semen is stored at unregulated ambient temperatures, typically at 5 x 10(6) spermatozoa per dose, and inseminated within 2.5 days of collection. In Experiment 1, the effect of storage temperature (5, 15, 22, 32 degrees C and fluctuations (Flux) between these temperatures) on progressive motility, viability, acrosomal status, DNA fragmentation and osmotic resistance was assessed. In Experiment 2, the field fertility of liquid semen at 5, 4 and 3 x 10(6) spermatozoa per dose, up to Day 2 after collection, was assessed in comparison to frozen-thawed semen at 20 x 10(6) spermatozoa per dose (n = 35 328 inseminations). In Experiment 1, storage at 15 degrees C resulted in the highest progressive motility (P < 0.01). The osmotic resistance of spermatozoa declined with duration of storage; however, after Day 3 this decline was reduced in the 5 degrees C and Flux 15 degrees C treatments (P < 0.01). In Experiment 2, the non-return rate of liquid semen stored at 4 and 3 x 10(6) spermatozoa per dose on Day 2 of storage was reduced in comparison to frozen-thawed semen (P < 0.01). In conclusion, liquid semen is versatile between storage temperatures of 5 and 22 degrees C, but demonstrates reduced fertility on Day 2 of storage at lower sperm numbers in comparison to frozen-thawed semen.