A study was conducted near Garden City, KS, under irrigated conditions to determine the effect of full-season Palmer amaranth infestation on corn water use efficiency and light interception in a fully developed corn canopy. Palmer amaranth at densities of 0, 0.5, 1, 2, 4, and 8 plants m−1 was established at corn planting in 1996 and 1997 and at two locations in 1998. Soil water was monitored 240 cm deep in 30-cm increments with a neutron probe each year and at each location every 10 d. Photosynthetic photon flux was measured in 1997 and 1998 by using a circular and a linear quantum sensor for above canopy and in four 50-cm increments for within canopy, respectively. Palmer amaranth reduced corn yield from 11 to 91% as density increased from 0.5 to 8 plants m−1. Water use efficiency of corn declined with increased Palmer amaranth density. Regardless of Palmer amaranth density, soil water extraction was greatest in the top 30 cm of the soil profile. The pattern of corn leaf area distribution was similar across Palmer amaranth densities, with 15, 70 to 75, and 5 to 15% of the total leaf area occurring 1.5 m, 0.5 to 1.5 m, and 0 to 0.5 m above the ground, respectively. In weed-free corn, over 60% of light was intercepted from 0.5 to 1.5 m above the ground. In contrast, in mixed canopies 60 to 80% of light was intercepted 1 m above the ground, where 80% of Palmer amaranth leaf area was concentrated. Under the conditions of this study, water was not a limiting factor. The effect of Palmer amaranth density on total light interception was not significant. However, within each treatment, light interception at different heights differed, emphasizing the importance of evaluating the vertical distribution of light through the canopy to assess the effect of weed height on light competition.