The area of the Montseny mountain whre the population of Rana temporaria lives is described. That mountain is part of the Catalonian Prelitoral System, so that it is clearly influenced by the mediterranean climate. Nevertheless, the higher points of the mountain are inhabited by some population of species evidently european as R. temporaria. For this species, the locality is the meridional distribution limit in the NE. of the Iberian Peninsula. The estimate of population size and population density have been calculated using several capture-recapture methods. Joiley's method has yielded a value of 820 individuals distributed in an area of 2.5 Ha. Jackson's negative method gave a total of 1333 individual and its positive and modified application yields a total of 894 individuals. The latter method is the most adequate to the characteristics of the sampling done. These results, to which we have to refer as the number of adult individuals, show that the population is moderately high. The sex-ratio is highly favourable to males, since considering al1 captures and observation data, it has been obtained a value of 0.28, giving in consequences the mate of al1 the females.