In spite of decades of continuous improvement in dairy sanitation and dairy quality, post-pasteurization contamination (PPC) of fluid milk by gram-negative organisms remains a burden to many processors. PPC may cause quality issues to arise during cold storage, including off-flavors, off-odors, changes in texture, and changes in color. PPC reduces the shelf-life of milk and results in increased waste. We collected finished product samples of pasteurized milk from 10 northeastern United States fluid milk plants 4 times over the course of 10 mo. To assess the prevalence and diversity of PPC, 280 samples were analyzed for total bacterial counts, coliforms, Enterobacteriaceae, and total gram-negative bacteria over 21 d of shelf life. Predominant organisms from all tests were identified and subtyped using 16s DNA sequencing. A trained sensory panel evaluated each sample at d 21 of shelf life. As has been demonstrated previously, PPC is highly prevalent in fluid milk, and 49% of samples contained heat-labile gram-negative bacteria that were most likely introduced post-pasteurization. Samples predominated by gram-negative organisms had more severe sensory defects than those contaminated with gram-positives. Coliform and Enterobacteriaceae tests, commonly used for PPC monitoring in the United States and the European Union, respectively, were unable to detect Pseudomonas, Acinetobacter, and several other gram-negative genera, which were the predominant organisms isolated in 38% of samples. Crystal violet tetrazolium agar, the standard for total gram-negative detection in dairy products, detected these organisms in 99% of cases. Using 16s sequence typing, we found evidence that certain bacterial strains responsible for PPC may persist in milk processing plants for several months. The distribution of 16s sequence types within a single plant provided a tool for pinpointing the processing steps at which contamination may have occurred. Subtyping methods are a potentially useful tool for fluid milk processors, as they may assist in the tracking of spoilage issues and the investigation and elimination of contamination sources within the plant.