Incubation temperature influences the phenotype of the hatchling turtles. The aims of the present study were to investigate the daily fluctuations in temperature to which eggs of the freshwater turtle Elusor macrurus are exposed to in the wild and examine how these fluctuations may affect the phenotype and performance of the hatchlings. Eggs in the wild experienced an overall mean daily fluctuation of 5.7°C throughout the incubation period, but on particular days, the variation was as low as 2°C and as high as 22°C. Fifty-four eggs were collected from the wild and incubated in the laboratory at one constant (28°C) and two fluctuating (28 ± 3 and 28 ± 6°C) thermal regimes. Egg mass, incubation length and hatching success (89%) were similar for the 28 and 28 ± 3°C groups, whereas the 28 ± 6°C group only had a 5% hatching success, and the incubation length was 10 days longer. Upon hatching, there was no significant difference in body mass or straight carapace length between the 28 and 28 ± 3°C groups, and within the first 8 weeks of hatching, there was no significant difference in growth rate, self-righting time, crawling speed and swimming performance. A single survivor from the 28 ± 6°C group had a body mass that was 27% less compared with the other two groups and it did considerably poorer in all the performance tests. The study findings illustrated that daily fluctuations in incubation temperature up to 6°C had no effect upon hatchling E. macrurus phenotype, but there was a limit (12°C) by which the extent and recurrence of these fluctuations became detrimental. These thermal regimes are not yet apparent in the wild but will occur within the geographical range of this species according to climate change predictions.