A description is given of the new clinical condition in fowl now known as EDS 76. This is characterised by the production of thin shelled, soft shelled and shell less eggs by apparently healthy birds. The effects on egg production are seen around 30 weeks of age and are seen either as an apparent fall in production or as a failure to obtain predicted production targets. Haemagglutinating viruses have been isolated from these flocks and evidence from experimental and field investigations suggest these are the aetiological agents of EDS 76. Biochemical and biological studies indicate that these viruses are duck adenoviruses. Epidemiological evidence suggests the virus is egg transmitted and that lateral spread between flocks is not important if care is taken whilst bleeding and vaccinating flocks. It is postulated that this virus may have been introduced into flocks by the use of a contaminated vaccine. Control in Northern Ireland has been by eradication of the virus from the basic breeding stock.