Technical Abstract: Irrigation waters have come under increasing scrutiny as a source of pathogenic microorganisms contaminating fresh produce. It is generally assumed that the microbial concentrations entering and leaving irrigation pipe networks are identical. However, this may not be true if biofilms form on the inner surfaces of irrigation pipes. The retention and release of pathogens in biofims is well documented in drinking water distribution systems, but very little data is available for irrigation systems. We examined the attachment and/or incorporation of total coliform, fecal coliform, and Escherichia coli bacteria into biofilms in new and used aluminum irrigation pipe. Water from a local creek in Maryland was used to conduct weekly irrigation events. Prior to each event, removable sections of pipe (coupons) were scraped to determine the extent of bacterial attachment; in addition, bacterial concentrations in residual water were determined. Substantial populations of coliform bacteria were found at the pipe surfaces. Old pipes had fewer attached or biofilm incorporated coliforms, and lower coliform concentrations in the residual water. High probabilities were found for fecal average coliform and total coliform concentrations being different between creek water and sprinkler water. These results have implications for monitoring and control of microbial quality of irrigation waters.