Commercially available fluctuating-temperature chambers are large and costly. This poses a challenge to experimental ecologists endeavouring to recreate natural temperature cycles in the laboratory because the large number of commercial chambers required for replicated study designs is prohibitively expensive to purchase, requires a large amount of space and consumes a great deal of energy. We developed and validated a design for economical, programmable fluctuating-temperature chambers based on a relatively small (23 L) commercially manufactured constant temperature chamber ($140US) modified with a customized, user-friendly microcontroller ($15US). Over a 1-week trial, these chambers reliably reproduced a real-world fluctuating (13·1–35·5 °C) body temperature regime of an individual frog, with a near-perfect 1 : 1 fit between target and actual temperatures (y = 1·0036x + 0·1366, R2 = 0·9977, 95% confidence interval for slope = 1·0026, 1·0046). Over 30-day trials, they also reliably produced a simpler daily fluctuating-temperature scheme (sine wave fluctuating between 10 and 25 °C each 24 h) and a range of constant temperature regimes. The design is inexpensive and simple to assemble in large numbers, enabling genuine replication of even highly complex, many treatment study designs. For example, it is possible to simultaneously examine in replicate chambers the responses of organisms to constant regimes, regimes that fluctuate following the means experienced by populations and regimes that exactly mimic fluctuations measured over any length of time for particular individuals that differ in behaviour or microhabitat use. These chambers thus vastly expand the pool of resources available for manipulative experiments in thermal biology and ecology.

validation_constant_10CTarget and actual temperatures logged for a 10 degrees Celsius constant-temperature validation scheme. We logged actual chamber temperatures every minute for 30 days.validation_constant_13CTarget and actual temperatures logged for a 13 degrees Celsius constant-temperature validation scheme. We logged actual chamber temperatures every minute for 30 days.validation_constant_16CTarget and actual temperatures logged for a 16 degrees Celsius constant-temperature validation scheme. We logged actual chamber temperatures every minute for 30 days. Add new data filevalidation_constant_19CTarget and actual temperatures logged for a 19 degrees Celsius constant-temperature validation scheme. We logged actual chamber temperatures every minute for 30 days.validation_constant_22CTarget and actual temperatures logged for a 22 degrees Celsius constant-temperature validation scheme. We logged actual chamber temperatures every minute for 30 days.validation_constant_25CTarget and actual temperatures logged for a 25 degrees Celsius constant-temperature validation scheme. We logged actual chamber temperatures every minute for 30 days.validation_ectotherm_regimeTarget and actual temperatures logged for a fluctuating-temperature validation scheme based on the body temperature regime of a small ectotherm. We logged actual chamber temperatures every minute for 7 days.validation_sinewaveTarget and actual temperatures logged for a sine-wave temperature validation scheme fluctuating between 10-25C over 24 hours. We logged actual chamber temperatures every minute for 30 days.