The purpose of this study was to examine the differences in performance and heart rate responses between a high heat outdoor condition (34.0°C, 64.1% humidity) and a temperate indoor condition (22.0°C, 50.0% humidity) during the 30-15 intermittent fitness test (30-15IFT). Eight highly trained Rugby Union players (28.1 +/- 1.5 years, 181.4 +/- 8.8 cm, 88.4 +/- 13.3kg) completed the 30-15IFT in two different temperature conditions. Dependant variables recorded and analysed included; final running speed of the 30-15IFT, heart rate (HR) at rest (HR rest), maximum HR (Max HR), HR recovery (HRR), average HR (HR ave) and sub-maximal HR corresponding to 25%, 50% and 75% of final test speed (HR 25%, HR 50% and HR 75%) and HR at 13 km[middle dot]h-1 (HR 13 km[middle dot]h-1). Greater running speeds were achieved when the test was conducted indoors (19.4 +/- 0.7 km[middle dot]h-1 vs. 18.6 +/- 0.6 km·h-1, p = 0.002, d = 1.67). HR ave and HR 13 km·h-1 were greater when the test was conducted outdoors (p <0.05, d > 0.85). Large effect sizes were observed for the greater HR at submaximal intensities (d > 0.90). The results of this study highlight the influence of temperature on 30-15IFT performance and cardiac responses. It is recommended that prescription of training based on 30-15IFT results reflects the temperature that the training will be performed in and that practitioners acknowledge that a meaningful change in assessment results can be the result of seasonal temperature change rather than training induced change.