Microgels exhibiting thermochromic behavior based on fluorescence resonance energy transfer (FRET) are prepared. The FRET microgels consist of poly(N-isopropylacrylamide) (PNiPAm) networks with fixed fluorescein and rhodamine moieties and exhibits volume phase transition (VPT) at 34-35 degrees C. A critical decrease in their sizes during the VPT enhances the efficiency of FRET between fluorescein as a donor and rhodamine as an acceptor. Therefore, emission from fluorescein (523 nm) and that of rhodamine (579 nm) is dominant at temperatures below and above the VPT temperature, respectively, when fluorescein is excited. We also prepare thermochromic core-shell FRET microgel exhibiting two-step color change. The microgels consist of a PNiPAm core and a poly(N-isopropylacrylamide-co-N,N-diethylacrylamide) shell and exhibit dual temperate-responsiveness at 19 and 33 degrees C. The fluorescence spectrum of the microgels also changes in two steps at these temperatures. (c) 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013