The ASTROCULTURE™ (ASC) middeck flight experiment series was developed to test subsystems required to grow plants in reduced gravity, with the goal of developing a plant growth unit suitable for conducting quality biological research in microgravity. Previous Space Shuttle flights (STS-50 and STS-57) have successfully demonstrated the ability to control water movement through a particulate rooting matrix in microgravity and the ability of LED lighting systems to provide high levels of irradiance without excessive heat build-up in microgravity.The humidity and temperature control system used in the middeck flight unit is described in this paper. The system controls air flow and provides dehumidification, humidification, and condensate recovery for a plant growth chamber volume of 1450 cm3. It provides the ASC flight units with a level of environmental control equivalent to most terrestrial plant growth chambers and has been shown to be capable of controlling humidity levels in the chamber to within 5% relative humidity over a range of 50% to 90% RH and air temperature levels in the chamber within 2°C. However, in this prototype unit, control is limited to the range of 24°C to 35°C. A prototype system was tested with no plants on STS-60, and a flight unit is currently scheduled to be tested on STS-63 in which the first experiments with plants will be conducted.