The stomata of three-month-old seedlings of Theobroma cacao L. var. Catongo were more open in high relative humidity (RH) (76 to 89%) than in low relative humidity (39 to 62 %). In both regimes, stomata closed gradually during the day, with the rate of closure accelerating in the late afternoon. Transpiration rate (TR) was correspondingly high early in the day and low late in the day. Average leaf diffusive resistance (r1) was 26% lower at the high RH. Nonetheless, TR was generally higher for plants in the low RH, because of the much greater vapour pressure gradient between the leaf and air. Abruptly lowering the RH at noon rapidly increased (r1) and increasing RH decreased r1, In another experiment conducted in constant high or low RH regimes, r1 was lower, the rate of net photosynthesis (Pn) was higher, leaf water potential (Φ) was lower (more negative), and TR was lower in the high RH regime. Water-use efficiency (WUE) was higher in high than in low RH. The relationships between Pn and r1 were identical at high and low RH. Thus, differences in TR and WUE at high vs low RH were a direct result of variations in vapour pressure deficit (VPD) between the two humidity regimes. Stomatal opening and closing reflected direct effects of humidity on guard cells rather than responses to changes in bulk leaf Φ. In addition, root to leaf hydraulic conductivity apparently was greater at low than at high RH.