Presently feed used for ruminants consists of forages as well as agricultural and industrial by-products, etc. Ruminants possess a special ability to degrade many substances including toxic substances due to their unique rumen microorganisms which are not present in monogastric animals. The ability of ruminal microorganisms to modify toxic and non-toxic substances is well established. This ability which is mostly favourable for the host animals may sometimes be detrimental. Tropical countries are known for their large diversity in flora and fauna, including plants with toxic properties and microorganisms with a wide adaptability. The emphasis of this review will be placed on the microbial degradation of forages with low digestibility in tropical countries. The discussion will be focused on the possible improvement of the nutrition of ruminants in tropical countries.