Asplenium platyneuron growing at two locations (outer sunlit location and inner shaded location) in a low brick and masonry wall in upper Manhattan (New York City) was studied to assess the effects of the two different microenvironments on the ecophysiology of the two fern populations. The intensity of illumination at the inner location was ca. 40% of the intensity at the outer location, where the photosynthetic active radiation (PAR) at mid-day was ca. 1,484 µmol m^–2 s^–1. Generally, root space temperatures tracked air temperature, varying from 26 to 31 °C for the outer location, and 25 to 28.7 °C for the inner location. Based on prior published literature with other fern species, we hypothesized that samples of A. platyneuron growing on the outer surface exposed to maximum illumination would exhibit higher maximum photosynthesis rates, higher respiration in the light, and smaller leaves compared to the less illuminated samples growing on the inner surface. The results supported our hypothesized relationships. Mean leaf length (cm) was longer for leaves of plants at the shaded inner location (30 ± 1.1) compared to those of plants at the more illuminated outer location (9 ± 1.2). The maximum photosynthetic rate (A_max) for the outside sample of ferns (6.3 ± 0.25) was statistically significantly different from the A_max for the inside sample (4.3 ± 0.72) as hypothesized. The mean respiration in the light was significantly larger for the ferns growing outside (-0.52 ± 0.03) compared to those inside (-0.36 ± 0.04). Additionally, light response (A–Q) curves and relevant other physiological evidence are presented for ferns at the two sampling sites.