Aims: To investigate the effects of fluorescent light intensity, sunlight intensity and temperature on photoreactivation of Escherichia coli after low-pressure (LP) and medium-pressure (MP) ultraviolet (UV) disinfection. Methods and Results: Two E. coli strains were irradiated with LP and MP UV lamps, and exposed to various fluorescent light (0-23 kLux) and sunlight intensities (1-80 kLux), and temperatures (4-50 degrees C). Escherichia coli concentrations were enumerated at hourly intervals to determine photoreactivation rates and final photoreactivation levels. Higher photoreactivation rates and levels were observed with increasing fluorescent light intensities, while high sunlight intensity (> 12 kLux) caused a one-log decrease in E. coli concentrations. When exposed to near-optimum growth temperatures (23-37 degrees C), photoreactivation levels were higher than those with too high (50 degrees C) or too low (4 degrees C) temperatures. Overall, photoreactivation following MP UV disinfection was lower than that following LP UV disinfection. Conclusions: Photoreactivation of bacteria following UV disinfection can be a problem in tropical countries where sunlight is abundant and temperatures are high, unless high sunlight intensity is present or if MP UV disinfection is employed. Significance and Impact of the Study: With the increased use of UV disinfection, it is imperative that photoreactivation be taken into account in the design of reactors based on site-specific conditions of temperature and light intensity exposure.