A case of 38 year old man who worked with organochlorinated and Parathion during 5 years is reported. His follow-up was up to 2 years. The onset of the disease was characterized by cholinergic signs, headache, loss of weight, trembling, miokimias, fasciculations, ataxia, myotonic phenomena (in hands only) and motor sensitive peripheral polyneuropathy (affecting the lower limbs symmetrically). Low concentrations of blood cholinesterases confirmed the etiology. Myotonic phenomena disappeared spontaneuosly 6 months after the initial observation. One year later, the concentration of erytrocyte acetilcholinesterase was found to be low and plasma cholinesterase was normal, suggesting that the patient was carrier of a congenital deficiency of acetilcholinesterase. in literature relationship between myotonia and intoxication due to organophosphorus was not found. The whole clinical picture, cholinergic symptoms, transitory myotonic phenomena and spontaneous motor activity could be explained by an excess of acetilcholine. Eletromyography (EMG) in the first observation showed neuromuscular transmission blocking characterized by deficiency or absence of voluntary activity, unexcitability of fibular nerves, with fibrilations and positive peaks as described previously with Mipafox (another organophosphorus agent). During 2 years of observation numerous end-plates potentials of muscular fibres persisted in the EMG. A progressive increase in voluntary activity showed by unit motor potential of almost normal amplitude and very increased duration was observed. No potentials of reinnervation were noted. The results of EMG were explained as disturbances of neuromuscular transmission associated with moderate signs of denervation The Eaton-Lambert's test and the stimulation of a single unit motor potential confirmed disorder of neuromuscular synapses. The histochemistry of brachial biceps showed scattered atrophic and angulated type I and II fibres. Teased-fibres preparations showed nerve fibres with B, C, and G alterations as defined by Dyck et al. indicating axonal degeneration. These results were according to velocity of sensitive conduction. The conduction velocity of fibular nerves was strongly delayed during all the evolution indicating serious disorders of motor nerves myelin. We think that intoxication by organophosphorus should not be expressed as an identical alteration of both motor and sensitive nerves. The association of myotonic phenomena with such clinical, electrophysiological and histological picture is characteristic of the Isaac's syndrome model with known etiology.