1. Three host-independent (H-I)Bdellovibrio species are stimulated in growth by addition to the medium of cell-free host extracts. A mixture of fifteen labeled amino acids was used to follow incorporation of counts into trichloroacetic acid (TCA) insoluble material. No label was recovered in the nucleic acids. 2. LabeledEscherichia coli RNA, protein, andAeromonas DNA preparations were hydrolyzed byBdellovibrio extracellular enzymes. The hydrolyzed protein or nucleic acids were added to a host free medium and inoculated withBdellovibrio. The amount of uptake and TCA precipitable counts in stationary phase cells was then determined. From these studies it has been demonstrated thatBdellovibrio extracellular enzymes hydrolyze host macromolecules to products which are subsequently used in the biosynthetic pathways ofBdellovibrio. 3. It is also suggested thatBdellovibrio cannot synthesize their own nucleotide bases, since neither14C-glycine nor3H-aspartic acid was incorporated into hot TCA soluble material.