Aim: To develop an inactivation kinetic model to describe ultraviolet (UV) dose-response behaviour for micro-organisms that exhibit tailing using two commonly referenced causes for tailing: physical shielding of micro-organisms and phenotypic persistence. Methods and Results: Dose-response data for Escherichia coli, Mycobacterium terrae and Bacillus subtilis spores exposed to UV radiation were fit to the phenotypic persistence and external shielding (PPES) model. The fraction of persistent micro-organisms in the original population (Npersistent/Ntotal) that exhibited reduced sensitivity to UV radiation was estimated by the PPES model as approx. 10(-7), 10(-5) and 10(-4) for E. coli, B. subtilis spores and Myco. terrae, respectively. Particle shielding effects were evaluated for Myco. terrae and resulted in additional reduction in UV sensitivity. Conclusions: Tailing occurred in laboratory experiments even when clumping and shielding were eliminated as major factors in UV resistance, suggesting that phenotypic persistence in addition to shielding may be important to consider when evaluating dose-response curves for disinfection applications. Significance and Impact of the Study: The PPES model provides a mechanistically plausible tool for estimating the dose-response behaviour for microorganisms that exhibit tailing in dispersed and aggregated settings. Accurate dose-response behaviour (including the tailing region) is critical to the analysis and validation of all UV disinfection systems.