The present study investigated how increasing silica content modifies the chemical and morphological characteristics and microbial degradation of rice straw. Two rice straws grown on culture media with different silica contents were collected and analysed for chemical and morphological composition and ultrastructure. Silica content (g/kg DM) of the two straws was high (98.0, straw H) and low (0.0, not detected, straw L). The ground (2 mm) straws were incubated in the rumen of three ruminally cannulated cows for 4, 8, 16, 24, 48 and 96 h for in sacco degradability measurements. The main fractions (stem, leaf sheath and blade) of each straw were incubated for 24 It. Kinetics were fitted to an exponential model with lag time. Scanning electron microscopy was used to examine intact fragments of stems and fragments incubated for 6, 24, 72 and 96 h in cow rumen. X-ray analysis was carried out on non-incubated fragments for silica mapping. A reduction of stem content from 320 to 270 g/kg straw, and an increase in leaf sheath content from 360 to 400 g/kg straw, were found for an increase in silica content from 0 to 100 g/kg DM. Chemical composition differed little between straws H and L, except for ash and silica contents. The ash originated mainly from the leaves and increased with silica content from 70 to 140 g/kg DM. The DM degradability was lower in the H than in the L straw. This reduction came mainly from a lower non-NDF component degradation in straw H. Stems were more degraded than leaf fractions for DM, NDF and non-NDF components; their degradation was depressed in the H straw. X-ray analysis showed that the deposition of silica on the stem epidermis was different, with a thick layer on H stems and no deposition on L stem surfaces. As a consequence, microbial colonization and degradation were faster on the epidermis of straw L. Epidermal silica did not hinder degradation from the internal cavity.