Greenhouse and growth chamber experiments were conducted with the following objectives: (a) to study the effects of N-1-naphthylphthalamic acid (naptalam) on shoot and tuber production, rhizome transformation, and growth of yellow nutsedge (Cyperus esculentus L.) plants; (b) to study the effects of N-phosphonomethyl glycine (glyphosate) on nutsedge plants at different stages of growth and on tubers that are produced after glyphosate application; and (c) to investigate the influence of naptalam on glyphosate toxicity to yellow nutsedge plants. The application of naptalam to yellow nutsedge plants increased the number of new shoots produced by the weed when the herbicide was applied to the soil, through a liquid growth medium, or as a topical spray. The increase in the number of new shoots in naptalam-treated plants was due to the transformation of rhizomes to shoots and to an increase in the number of rhizomes produced. Growth of naptalamtreated plants was inhibited. Split topical applications of naptalam proved to be more effective in producing new shoots than single topical applications. There was no difference in the number of new shoots produced by plants treated with either single or split soil applications except at the highest concentration where the split application resulted in more new shoots than the single application. Results of experiments conducted in growth chambers showed that naptalam- treated plants produced more new shoots than untreated plants when grown under a 20-hour photoperiod. Growing the plants under short day conditions (10 to 12-hour photoperiod) induced tuber production but this was inhibited by naptalam application. The application of nitrogen to naptalam- treated plants grown under long day conditions increased the dry weight of new shoots produced by the plants. Naptalam was more effective in inducing the production of new shoots in younger plants than in older ones. Glyphosate was more effective for reduction of shoot growth of younger yellow nutsedge plants than of older ones. There were no significant differences in dry weights of shoots of the plants when split and single topical applications were compared. Tubers produced by glyphosate-treated plants sprouted less than tubers produced by the untreated plants. Glyphosate appeared to make the tubers dormant at the lower rates and to kill many of them at the higher rates. However, over 50 percent of the tubers were not killed and were still capable of regeneration. It was more advantageous to apply glyphosate to yellow nutsedge plants after naptalam treatment than to apply the two herbicides simultaneously. Simultaneous application of naptalam and glyphosate delayed the production of naptalam-induced new shoots because glyphosate was preferentially translocated to the rhizomes that were being transformed to shoots and inhibited their growth. On the other hand, application of glyphosate 1 to 4 weeks after naptalam treatment resulted in a synergistic effect on the yellow nutsedge plants. The application of naptalam followed by glyphosate was superior to either glyphosate or naptalam alone in reducing the number of tubers produced by regrowth of yellow nutsedge plants. There was a marked reduction in the total number of rhizomes and tubers of plants treated with naptalam followed by glyphosate indicating that most of the rhizomes that were transformed to shoots by the naptalam treatment were killed by the glyphosate spray. Based on the findings of this study, a spray program using naptalam and glyphosate to provide improved control of yellow nutsedge appears possible.