Translation of fdhF mRNA, encoding formate dehydrogenase H, requires the context-specific insertion of selenocysteine at an opal codon. We have cloned the Tn10 insertion previously shown to block fdhF transcription in Salmonella typhimurium and shown that it lies in selD, which encodes phosphoselenate synthetase. A spontaneous mutant of the selD::Tn10 strain that expresses an fdhF::lacZ protein fusion (where lacZ is fused after the opal codon) was isolated and characterized. Suppression showed the same context requirement as previously reported for SelB insertion of selenocysteine. The suppressing mutation was mapped by P22 co-transduction to the 74–75 min of the Salmonella typhimurium genome.