SYNOPSIS The mean and variance of environmental temperature are changing as a consequence of human activities. Ectotherms are sensitive to these temperature changes in the short term, typically displaying a unimodal response of most biological rates to temperature (thermal performance curves; TPCs). Many organisms, however, may acclimate or evolve in response to new temperature regimes. In particular, population growth rate TPCs (r TPCs) reflect the ability to maintain positive growth under a range of temperatures, and therefore shifts in r TPCs due to acclimation are fundamental to our understanding of how ectotherms will respond to changes in climate. Here, we derive a model for r TPCs rooted in temperature dependent metabolic rate (through enzyme kinetics and activity). We then use this model to interpret the effects of acclimation to different temperatures on r TPCs of the protist Paramecium bursaria. Intermediate acclimation temperatures generally resulted in higher upper critical thermal limits, thermal optima, maximum population growth rate, and the area under the TPC. Lower critical thermal limits increased linearly with acclimation temperature, causing a decrease in thermal breadth with increased acclimation temperature. Thus, rather than showing improved performance at the acclimation temperature, P. bursaria appeared to pay a price at all temperatures for acclimating to higher temperatures. The fits of our data to our model also suggest that changes in the structure and function of metabolic enzymes may underlie the changes in the TPCs. Specifically, our results suggest that both the delta heat capacity and delta enthalpy of formation of metabolic enzymes may have increased with acclimation. Since these two factors are correlated across acclimation temperatures, our data also suggest potential trade-offs that may constrain changes in TPCs.