The biochemical basis for a cysteine requirement in Salmonella pullorum strain MS35 is presented. Before determining the missing biochemical functions, it was established that the assimilatory sulfate-reducing pathway for this species is an inorganic one in which 3′-phosphoadenylylsulfate (PAPS), sulfite, and sulfide are intermediates. A requirement for 2′- and 3′-adenosine monophosphate was found for in vitro synthesis of PAPS, possibly because 2′- and 3′-adenosine monophosphate inhibits endogenous nucleases that destroy PAPS. The cysteine requirement of strain MS35 was attributed to a defect at 37 C in sulfate permeation and temperature sensitivity in sulfite reduction. At 25 C, sulfite was metabolized to sulfide. A novel property of sulfate-utilizing revertants was their unselected ability to assimilate thiosulfate sulfur at 25 C but not at 37 C.