Moisture transfer characteristics of four types of bale covering materials for gin universal density cotton bales were evaluated during a one year period. Twelve bales (four coverings and three replications) were ginned at low moistures (3.4%, wet basis), covered with materials with different levels of permeability and stored at 70 F and 80% relative humidity. Bales were weighed periodically to establish the weight change as a function of time. Bale covering materials were a 6 mil thick, linear low density polyethylene bag with perforation holes (LLDPE); woven polypropylene, spiral-sewn bag, 1050 denier with alternating 1.75" by 1.75" coated and uncoated strips (WPP1050), woven polypropylene, spiral-sewn bag, 840 denier with alternating 1.75" by 1.75" coated and uncoated strips (WPP840), and woven polypropylene, spiral-sewn bag, 840 denier with alternating 11" coated and 1.5" uncoated strips (WPPII). The coatings were extruded polypropylene. At the end of 161 days of storage the WPP1050 and WPP840 bales were removed from storage because they had equilibrated with the environment. The remaining six bales were retained for a total of 378 days. At the end of the 161 day storage period, bale weight gains were 10.3, 21.5, 25.7, and 26.1 pounds, respectively, for the LLDPE, WPP11, WPP1050, and WPP840 coverings. At the end of that period the LLDPE-covered bales had not reached equilibrium but the WPP11-covered bales had. The WPP11-covered bales reached equilibrium at about 210 days of storage. Thus, the permeability of bale covering materials is related directly to the rate of weight gain of cotton bales.