Myotonia congenita is a genetic muscle disorder characterized by clinical and electrical myotonia, muscle hypertrophy, and stiffness. It is inherited as either autosomal-dominant or -recessive, known as Thomsen and Becker diseases, respectively. These diseases are distinguished by the severity of their symptoms and their patterns of inheritance. Becker disease usually appears later in childhood than Thomsen disease and causes more severe muscle stiffness and pain. Mutations in the muscular voltage-dependent chloride channel gene (CLCN1), located at 7q35, have been found in both types. We report here the case of a Moroccan consanguineous family with a myotonic autosomal-recessive condition in two children. The molecular studies showed that the patients reported here are homozygous for mutation p.Gly482Arg in the CLCN1 gene. The parents were heterozygote carriers for mutation p.Gly482Arg. This diagnosis allowed us to provide an appropriate management to the patients and to make a genetic counselling to their family.