Field studies were conducted in 2010 and 2011 at Belle Glade, FL to evaluate the influence of phosphorus (P) application (98, 196, and 293 kg P ha^-1) on the critical period of weed control (CPWC) in lettuce. Natural populations of mixed weed species were allowed to interfere with lettuce in a series of treatments of both increasing duration of weed interference and duration of weed-free period imposed within 98, 196, and 293 kg P ha^-1 levels added to the soil. The beginning and end of the CPWC for each P level based on a 5% acceptable marketable fresh lettuce yield loss level was determined by fitting log-logistic and Gompertz models to represent increasing duration of weed interference and duration of weed-free period, respectively. The beginning of the CPWC in lettuce was estimated to be 2.2, 2.3, and 2.9 WAE at 93, 196, and 293 P kg ha^-1, respectively which corresponded to the four- to six-leaf stage of lettuce development. The end of the CPWC in lettuce was 6.8, 5.7, and 5.2 WAE at 93, 196, and 293 P kg ha^-1, respectively which corresponded to the cupping to heading stage of lettuce development. Beginning of the CPWC was delayed at high P level (293 kg P ha^-1) while the end of the CPWC was hastened at high P level. Our study shows that reduced levels of P fertilization in lettuce will result in the need for more intensive weed management practices to attain acceptable yields.