Ufra, caused by the stem nematode,Ditylenchus angustus(Butler, 1913) Filipjev, 1936, is a serious disease of deepwater rice in southern Bangladesh. It was studied between 1977 and 1979 as part of a joint project with the Overseas Development Administration at the Bangladesh Rice Research Institute. Three symptom types are distinguished according to the extent of panicle emergence: Ufra1 (none), Ufra2 (partial) and Ufra3 (complete). All three represent total loss from that panicle. The number of Ufra2 as a percentage of the total number of panicles present in a specified area is selected as a disease index (UfraII). A function relating yield to the level of UfraII is proposed on the basis of selective samples from a field with a well-defined ufra patch. This function is compared with another derived from random samples in different fields along a linear transect through an area where ufra is endemic. Loss of panicle density, i.e. panicles/m2, is identified as a major component of yield loss, approximately equal to the loss associated with all three symptom types added together. Although severe and widely distributed throughout the southern part of the deepwater rice region in 1977 and 1978, it was very difficult to find in 1979 when there was a spring drought. Ufra is worse in wet years, in areas which flood early, and where the crop is harvested late. The length of the over-winter decay phase appears to be a critical factor regulating the survival of the pathogen from one season to the next. This suggests an approach for ufra control.