Coco fibres are used as a wrapper of plastic drain pipes. An investigation has been made of the decomposition of these fibres in soil. Unaffected and partly decayed fibres were washed and placed on cellulose-containing agar plates. Unaffected coir rarely showed growth. Around the partly decayed fibres nearly always fungi developed. A number of these, cellulose-decomposing fungi were isolated and identified. Total reducing sugars, hexoses, lignin and nitrogen were estimated in unaffected and partly decayed coir samples. Especially the nitrogen content correlated with the stage of decomposition of the fibres. A laboratory method is described for studying the breakdown of coir under conditions more or less equal to those in the field.