Adequacy of the cultivated cotton varieties 'IAC 17' and 'IAC 18' for mechanical harvesting and the possibility to improve their performance by using the plant growth regulator Cycocel, were studied at Leme, State of São Paulo, Brazil, in the 1978/79 and 1979/80 crop seasons. In the average of the two years, about 89% of the produced cotton was harvested, at one time by a John Deere 9900 two-row picker type machine. Field losses ranged from 7% (in the normal year of 1978/79) to 16% (in 1979/80), when unfavorable climatic conditions prevailed. Grade of seedcotton was 1 to 2 points lower in machine than in hand harvested product. Field losses were about 2% greater for 'IAC 17', the differences between varieties being due to cotton fallen on the ground as well as to that left in the plants. Damage in grade was about the same for both varieties in the normal year, but considerably higher for 'IAC 17' under adverse conditions. Application of 50g/ha of Cycocel increased percentage of cotton harvested mechanically, but this occurred only under adverse climatic conditions. Otherwise, even in the unfavorable year, no beneficial effect on cotton grade could be attributed to this product. Except for an inconsistent tendency for lower fiber strength (Pressley 1/8 inch gauge), fiber properties were not affected by mechanical harvesting.