The rpoS gene of Gram negative bacteria encodes an alternative sigma factor controlling the expression of a wide variety of genes involved with stress survival, nutrient starvation, and rapid changes in environmental conditions. Bdellovibrio bacteriovorus is a bacterium capable of attacking, invading, and parasitizing other Gram negative bacteria; this unusual microbe alternates between a free swimming "attack" phase in search of prey cells and the intraperiplasmic "parasitic" phase growing at the expense of the host cell. Bdellovibrio clearly experiences many changes in its environment during this alternation, including nutrient status (starvation outside host cells, adequate nutrition within host cells) and possible stressful conditions (osmotic stress within the periplasm of host cells). As a first step toward analysis of the genetic and biochemical strategies used by Bdellovibrio to support its unusual prokaryotic lifestyle, the rpoS gene of Bdellovibrio has been isolated, using a unique functional complementation assay with an E. coli reporter gene construct. We hope that analysis of the rpoS gene of Bdellovibrio will yield insights into the types and numbers of gene products (and the regulation of such genes) used by Bdellovibrio during its transition from "attack phase" to "parasitic phase" life.