Objective of the study is to assess the prevalence of Connexin 26 (GJB2) mutation in patients with congenital nonsyndromic sensorineural hearing loss in Bulgarian population. Study design is done prospectively. Patient inclusion criteria for this study were diagnosis of congenital nonsyndromic hearing loss, and absence of potential sibling relationships between patients included in the study (anamnestic pedigree for at least three generations). Patients were excluded from the study group if one of the following conditions were present: secondary hearing loss (cytomegalovirus, rubella, meningo-encephalitis, mastoiditis, other infections, posterior fossa tumors, etc.), exposure to drugs or other prenatal or perinatal etiology of deafness, and congenital syndromic hearing loss. Genomic DNA samples from whole blood were tested with sequence analysis for mutations in the coding region of the GJB2. Results state that 51 patients were analyzed for GJB2 mutations. Twenty of the patients (39%) with mutant alleles were homozygous for the c.35delG mutation (c.35delG/c.35delG) and four patients (8%) presented as heterozygotes (c.35delG/WT). In one patient, who carried a heterozygous mutation c.35delG, a second mutation was found-312del114. Additionally, in two other patients were discovered the mutations Trp24X (W24X) and, respectively, Arg127His(R127H), both in heterozygous states. From the whole study group there was only one patient with compound heterozygous genotype-p.Leu90Pro(L90P)/p.Ile121Asn. The latter one has never been reported in the literature so far. In conclusion, this study determines the importance of connexin 26 mutations in Bulgarian children with severe to profound congenital nonsyndromic sensorineural hearing loss, the prevalence of the different mutation variants and their relationship with the ethnical background of the patients. In addition, we report for the first time a novel mutation in the GJB2 gene.