The seasonal variation in δ13C values was measured in leaves from 17 upper canopy five mid- canopy and in four gap tree species as well as in five epiphyte and five vine species in a seasonally dry lowland tropical forest at Parque Natural Metropolitano near Panama City Republic of Panama. No seasonal variation was detected in the δ13C values of mature exposed leaves from either the upper or mid- canopy. However canopy position did influence the δ13C value. The mean isotopic composition of leaves from the mid- canopy was more negative than that of the upper canopy throughout the year. The δ13C value was also influenced by leaf development with juvenile leaves on average 1.5 per mil less negative than mature leaves. The five epiphyte species exhibited δ13C values that were typical of crassulacean acid metabolism (CAM). Codonanthe uleana with isotopic values of -19.9 to -22.1 per mil is only the second species in the Gesneriaceae reported to express CAM whereas values between -14.6 and -22.0 per mil indicate that Peperomia macrostachya can exhibit different degrees of CAM. The isotopic composition of exposed mature leaves from the vines showed little interspecific variation and was similar to the upper-canopy leaves of the trees.