Polymerase chain reaction (PCR) was used to amplify transcripts encoding cytochrome b ₅ from cDNA synthesised from RNA isolated from developing seeds of tobacco (Nicotiana tabacum L.). The sequence of the amplified products indicated that the clones encoded a second form of tobacco cytochrome b ₅, different from that previously characterised (Smith et al. 1994, Plant Mol Biol 25:527–537). Rapid amplification of cDNA ends (RACE)-PCR was used to amplify the 5 and 3 ends of the transcript. Northern blotting and RNAse protection assays of RNA samples isolated from different tobacco tissues indicated that this second cytochrome b ₅ form was expressed only in developing seeds. Therefore, it seems likely that this message is the product of a tobacco cytochrome b ₅ gene specifically expressed in seeds.