Intravenous and intraarterial catheters were examined microbiologically and morphologically. Bacteria or yeasts were recovered from 38 of the 63 catheters examined, and Staphylococcus epidermidis was present on 29 of the 38 colonized catheters. Examination of unused Teflon catheters ( Jelco ; Surgikos , Inc., Peterborough , Ontario, Canada) showed surface irregularities, and the examination of colonized intravascular catheters recovered from patients showed very extensive amorphous accretions on both their lumenal and external plastic surfaces. Detailed scanning electron microscope examination of the accretions on vascular catheters from which S. epidermidis had been isolated showed (ca. 0.8 micron) coccoid bacteria within confluent biofilms , in which they were enveloped by amorphous material. Transmission electron microscope examination of these same accretions revealed coccoid cells (ca. 0.8 micron) with a gram-positive cell wall structure living in fibrous matrix-enclosed microcolonies in spaces between squamous epithelial cells. Staphylococcus aureus biofilms were seen to contain coccoid cells (ca. 1 micron) in a very extensive amorphous matrix, and a Candida parapsilosis biofilm contained very large numbers of large coccoid cells (ca. 4.3 microns) in a fibrous matrix resembling fibrin. Cells of a Corynebacterium species appeared to form much less extensive matrix-enclosed microcolonies on the colonized plastic surface. These data indicate bacteria and yeasts colonize intravascular catheters by an adherent biofilm mode of growth on these plastic surfaces.