The gene frequency (q) of β-thalassemia (T) is more difficult to evaluate directly than the frequency of hemoglobin variants (V), Hb S and Hb C being the most frequent ones in Algeria. Among 150 subjects with a phenotype V, we identified 76 compound heterozygotes VT° (T° = β°-thalassemia) and 74 homozygotes VV: qT° is therefore practically equal to qV. Calculation based on the investigation of 54 subjects detected during the same period (33 VT among which 23 VT° and 21 TT among which 9 T°T°) yields qT = 1.43 qT = 1.43 qV. According to Cabannes , qV is equal to 0.0113, qT is 1.43 × 0.0113 = 0.0162. This indirect method, based on investigation of patients, gives a less precise evaluation of gene frequency than a direct method based on large population screening. This evaluation, however, is sufficient to estimate the incidence of β-thalassemia and its impact from a public health point of view.