A new form of congenital myasthenia is described. An infant whose foetal movements during pregnancy had been weak presented at birth with muscle weakness and multiple contractures of the lower limbs. The clinical course was characterised by myasthenic crises during febrile illnesses. Neurophysiological studies demonstrated a decremental response at 2-3 HZ stimulation; this effect was reversed by edrophonium iv. At the age of eight months, a biopsy from the soleus muscle showed a predominance of type I fibres and variation in fibre diameters was slightly increased. Ultrastructural studies of the motor endplates revealed a marked reduction of post-synaptic membrane lengths with paucity of secondary clefts. Signs of focal degeneration were absent. The picture was reminiscent of foetal neuromuscular junctions. A developmental disorder of the postsynaptic membrane was considered to be the basic cause of the neuromuscular transmission defect.