jRCaMP1a is a bright red genetically encoded fluorescent Ca[2+] indicator. It is a fusion protein composed of the calmodulin-binding M13 peptide, a circularly permuted red fluorescent protein and calmodulin. The circularly permuted fluorescent protein is based on mRuby and contains the amino acid substitutions T62C, Y112S and L298T relative to its progenitor RCaMP1h. jRCaMP1a provides an improved signal-to-noise ratio and may be used in combination with ChR2 (FBrf0232227).