This study aimed at measuring transcriptome-wide gene expression in four populations of the copepod T. californicus. Populations were exposed to two different thermal regime, a variable 20-28 degree and a constant 20 degree followed by a one-time stress at 28 degrees. RNA was extracted at 20 and 28 degrees for all populations from all thermal regimes. Transcriptome-wide gene expression was measured, and differentially expressed genes were determined for all populations for the different temperature combinations. The results show that each populations responds to the changes in temperature in a drastically different manner (in terms of gene expression), which can have implications regarding the way these and other organisms will respond to climate change.