Various methods exist for the measurement of plant water status. Plant breeders value methods that are fast and inexpensive lending themselves to the efficient evaluation of large segregating populations. Chlorophyll fluorescence is a parameter commonly measured by plant physiologists when studying plant response to changing water status. The objectives of this greenhouse study were [1] to determine if chlorophyll fluorescence measurements could differentiate 2 cotton genotypes grown under various water regimes and [2] establish the relationship between chlorophyll fluorescence and dry matter accumulation. Based on previous field experiments, 2 genotypes were selected according to their tolerance to drought as monitored via chlorophyll fluorescence. TAMCOT 22 was shown to be drought tolerant and TAM 89E-51 was shown to be susceptible. Two seeds were sown of each genotype in 1-gallon pots filled with Metro-Mix 200 (Sun Gro, Bellevue, WA). The plants were thinned to 1 plant per pot after germination. The experiment consisted of 5 water treatments and two replications. There were ten plants of each genotype per replication. 4 of the 5 water regimes called for the plants to be watered to field capacity every other day until the plants reached the 5-true leaf stage. The fifth treatment, or low water (LW) treatment, called for the plants to be watered at 27 percent of field capacity from the start of the experiment. At the fifth leaf stage irrigation was discontinued in the stop water (SW) treatment, irrigation was lowered to 27 percent of field capacity in the reduce water (RW) treatment, irrigation was continued in the control (C) treatment, and the water level was raised to 2.5 cm above the soil surface in the waterlogged (WL) treatment. An initial fluorescence measurement was taken just prior to establishing the 5 treatments. Leaf punches were harvested in early morning from the fifth true leaf of each plant and placed in 24-well plates half-filled with dH2O. After returning to the laboratory, leaf punches were transferred to moist filter paper lining a pyrex dish. The punches were covered with Glad wrap and an initial measurement was taking with an OS1-FL modulated chlorophyll fluorometer (Opti-Sciences, Hudson, NH). The punches were then incubated at 40°C and additional measurements were taken hourly for 5 hours. Under this procedure stressed plants maintain high levels of fluorescence after incubation due to carbohydrates that were not mobilized overnight. The opposite is true for non-stressed plants. Fluorescence was measured 6 times over a two-week period. At the conclusion of the experiment number of leaves was recorded and plants were harvested at the cotyledonary node for the measurement of fresh and dry weights. Variance analysis indicated that genotypes and treatments were significantly different at the 0.05 and 0.0001 probability levels, respectively. For both genotypes, the stop water treatment had the highest fluorescence values and the control treatment had the lowest values at the conclusion of the experiment. The remaining treatments were intermediate and were not distinguishable from one another. A negative correlation (r²=0.32) was found between fresh weight and chlorophyll fluorescence.