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Different night temperature regimes were maintained throughout the boll development period on cotton growing in the field during the 1963, 1964, and 1965 seasons. Both fruiting and boll development were found to be closely associated with night temperature. Decreased night temperature resulted in the formation of more flowers and, in some cases, increased fruit set. Relative fruitfulness increased as night temperature decreased between the interval of 21 and 8.8 C and 21 and 14.4 C with ‘Paymaster’ and ‘Acala’ varieties respectively. Temperatures above 19.4 and below 13.3 C reduced fruiting index. Rate of boll development was inversely related to temperature, with a decrease of either day or night temperature resulting in slower boll dvelopment, thus, increasing boll periods. However, in this work night temperature was the dominant environmental factor associated with boll periods. In general, the Paymaster variety appeared to be better adapted to low night temperatures than the Acala variety.

Different night temperatures were maintained through‐out the seed development period of two divergent varieties of cotton growing in the field during the 1963, 1964, and 1965 seasons. Viability and oil and nitrogen content of cottonseed of both varieties were found to be affected by the night temperature prevailing during the seed development period. Decreased night temperatures resulted in lower germination percentages and in reduced oil and nitrogen content of the seed. Differences were also noted between varieties and seasons, and an inverse relationship existed between oil and nitrogen content for both varieties at all temperature regimes.