Green fluorescent protein (GFP) has become an increasingly popular protein tag for determining protein localization and abundance. With the availability of GFP variants with altered fluorescence spectra, as well as GFP homologues from other organisms, multi-colour fluorescence with protein tags is now possible, as is measuring protein interactions using fluorescence resonance energy transfer (FRET). We have created a set of yeast tagging vectors containing codon-optimized variants of GFP, CFP (cyan), YFP (yellow), and Sapphire (a UV-excitable GFP). These codon-optimized tags are twice as detectable as unoptimized tags. We have also created a tagging vector containing the monomeric DsRed construct tdimer2, which is up to 15-fold more detectable than tags currently in use. These tags significantly improve the detection limits for live-cell fluorescence imaging in yeast, and provide sufficient distinguishable fluorophores for four-colour imaging. Copyright © 2004 John Wiley & Sons, Ltd.