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Much interest exists in the extent to which constant versus fluctuating temperatures affect thermal performance traits and their phenotypic plasticity. Theory suggests that effects should vary with temperature, being especially pronounced at more extreme low (because of thermal respite) and high (because of Jensen's inequality) temperatures. Here we tested this idea by examining the effects of constant temperatures (10 to 30 °C in 5 °C increments) and fluctuating temperatures (means equal to the constant temperatures, but with fluctuations of ±5 °C) temperatures on the adult (F2) phenotypic plasticity of three thermal performance traits – critical thermal minimum (CT min), critical thermal maximum (CT max), and upper lethal temperature (ULT 50) in ten species of springtails (Collembola) from three families (Isotomidae 7 spp.; Entomobryidae 2 spp.; Onychiuridae 1 sp.). The lowest mean CT min value recorded here was -3.56 ± 1.0 °C for Paristoma notabilis and the highest mean CT max was 43.1 ± 0.8 °C for Hemisotoma thermophila. The Acclimation Response Ratio for CT min was on average 0.12 °C/°C (range: 0.04 to 0.21 °C/°C), but was much lower for CT max (mean: 0.017 °C/°C, range: -0.015 to 0.047 °C/°C) and lower also for ULT 50 (mean: 0.05 °C/°C, range: -0.007 to 0.14 °C/°C). Fluctuating versus constant temperatures typically had little effect on adult phenotypic plasticity, with effect sizes either no different from zero, or inconsistent in the direction of difference. Previous work assessing adult phenotypic plasticity of these thermal performance traits across a range of constant temperatures can thus be applied to a broader range of circumstances in springtails. • Adult springtails exhibit similar thermal tolerance plasticity when exposed to constant versus fluctuating temperatures. • Thermal tolerance trait values increase as acclimation temperature increases. • Previous work using constant temperatures to estimate critical thermal limits can be considered reliable. [ABSTRACT FROM AUTHOR]