Résumé : In a sample of approximately 11 000 hypernuclei obtained from K--meson captures at rest in nuclear emulsion, eleven examples of the π+-mesonic decay of hypernuclei have been observed. From the ten which are attributed to the decay of4HeΛ, a value of (9±3)% is obtained for R, the ratio of π+ to π- mesonic decays for this hypernucleus. This result is larger than both that previously found experimentally and that predicted theoretically. © 1966 Società Italiana di Fisica.