The effect of moisture conditions on yield and microbial quality of iceberg lettuce was investigated. Iceberg lettuce cvs. Honcho II and Sahara grown at the Yuma Agricultural Center were evaluated for weight, diameter, microbial population, and postharvest quality, either following different irrigation termination schedules or before/after a rainfall event. Two trials were conducted with early (24 or 18 days before harvest), middle (16 or 8 days before harvest) and late (6 or 4 days before harvest) irrigation termination. Lettuce receiving the last irrigation 6–4 days before harvest showed increased weight and diameter, higher total aerobic bacteria and shorter shelf life than plants having the early irrigation termination. The plants receiving middle termination irrigation showed similar weight at harvest, lower total aerobic count and longer shelf life than plants receiving late irrigation termination. The effect of the field's moisture prior to harvest on quality was further evaluated with lettuce harvested before and 1, 2 and 7 days after a rainfall event. After rain, mesophilic bacteria population increased 1 log10 CFU/g in outer leaves and more than 2 log10 CFU/g in head leaves. The microbial population in outer leaves declined more rapidly, possibly due to more rapid drying and higher impact of sun UV light. The results from this study suggest that managing moisture conditions at harvest is important to enhance quality of lettuce. Although the potential decrease in weight produced with an early irrigation termination is a great concern of growers, it was shown in this study that excessively late preharvest irrigation of lettuce is not necessary to obtain maximum weight at harvest.