A previously undescribed nucleoside salvage pathway for NAD biosynthesis is defined in Salmonella typhimurium. Since neither nicotinamide nor nicotinic acid is an intermediate in this pathway, this second pyridine nucleotide salvage pathway is distinct from the classical Preiss-Handler pathway. The evidence indicates that the pathway is from nicotinamide ribonucleoside to nicotinamide mononucleotide (NMN) and then to nicotinic acid mononucleotide, followed by nicotinic acid adenine dinucleotide and NAD. The utilization of exogenous NMN for NAD biosynthesis has been reexamined, and in vivo evidence is provided that the intact NMN molecule traverses the membrane.