Listeria monocytogenes can persist in food processing environments, resulting in the predominant source of L. monocytogenes post-processing ready-to-eat (RTE) food contamination. Butt's "Seek and Destroy" strategy is a systematic method for finding sites of persistent growth ("niches"); it may reduce L. monocytogenes prevalence in RTE food and expedite detection and response to L. monocytogenes outbreaks. Applying this strategy, we sampled environments at two smoked fish plants to identify persistent L. monocytogenes ribotypes, and to identify and eliminate or manage niches. Persistence was measured with binomial statistics: one statistic compared ribotype recurrences to reference distributions; the second measured ribotype occurrences as a risk factor. Persistent ribotypes and persistence sites were identified to guide interventions. Poisson regression showed borderline decreases in L. monocytogenes isolation at both plants (p=0.026 and p=0.076). One niche on a food contact surface was eliminated; others were not. These methods should facilitate identification of microbial persistence. ii