The effect of two controlled, contrasting environments on the anatomy and cell wall degradability of leaf laminae was examined in a range of temperate and tropical grass species: Dactylis glomerata cv. S37, Festuca arundinacea cv. S170, Lolium perenne cv. S24, Phleum pratense cv. S48 (temperate); Bothriochloa barbinodis, Cenchrus ciliaris cv. Biloela, Chloris gayana cv. Pioneer and Eragrostis curvula (tropical). Environment I was cool and humid with a low light intensity and adequate water supply; Environment II was hot and dry with a higher light intensity and restricted water supply. Cell walls were isolated from leaf laminae and their degradability determined using a commercial fungal cellulase. The anatomy of the lamina of the eighth leaf on the main stem was examined to determine the possible anatomical basis for differences observed in cell wall degradability. Differences in cell wall degradability were found between species grown in the same environment, but there was no significant difference between the tropical and temperate species when considered as two separate groups. Environment had a significant but opposite effect on the tropical and temperate species: the cell walls of all the tropical grasses were more degradable in Environment II than in I, whereas in the temperate species they were less degradable. It was concluded that the low feeding value of tropical grasses is not caused by an inherently low degradability of cell walls in the leaf laminae, nor by the effects of a hot, dry environment on cell wall degradability. Anatomical features which could have accounted for differences between species in cell wall degradability included variation in the lignification of the parenchyma sheath cells and epidermis and in the size of girders, strands and marginal caps, as seen in transverse section. The increase in cell wall degradability in tropical. species in Environment II was due, at least in part, to an increase in the proportion of sclerenchyma fibres which were degraded by cellulctse. This type of sclerenchyma fibre had very thick walls and was only ii found in the tropical species. The decrease in cell wall degradability in temperate species could be accounted for by an increase in the thickness of lignified cell walls, and an increase in the resistance of epidermal and parenchyma sheath cell walls to degradation.