By introducing this work on brazilian nematodes under Procamallanus Baylis, 1923, the authors propose that Spirocamallanus Olsen, 1952, must be regarded only as a good subgenus based on the aspect of the buccal capsule. For this reason, Spirocamallanus should identify those worms bearing spiral thickenings, as well as similar structures inside their buccal cavity while Procamallanus may be applied every time the nematodes present a completely smooth capsule, lacking the above mentioned structures. Regarding to the early propositions in order to subdivide the genus using number and shape of the spicules as differential characters, the authors fully disagree with this point of view, which is rather a poor criterion, for the spicules are weakly chitinised in this group and for this reason they do not present enough strutural uniformity to be of great systematic significance. The authors deal here with three species: Procamallanus (Spirocamallanus) rarus Travassos, Artigas & Pereira, 1928, Procamallanus (Spirocamallanus) pimelodus and Procamallanus (Spirocamallanus) intermedius. The last two are proposed as new species. Procamallanus (S.) pimelodus is related to Procamallanus (Spirocamallanus) olseni Campanarouget & Razahihelissoa, 1965 and differs from it mainly by its caudal alae which shows a strongly muscular fringed rim. Procamallanus (S.) intermedius only comes nearer to Procamallanus (S.) rarus by possessing the bigger spicule with distal bifurcation and differs from it by the aspect of the buccal capsule which has more and thinner spiral thickenings. Of Procamallanus (S.) rarus, the female is described, a redescription of the male is presented and the very peculiar aspect of the bigger spicule is pointed out for the first time in the genus. This was also observed when dealing with Procamallanus (S.) intermedius, after examining the type of Procamallanus (S.) rarus. In this first part of the work the authors handled 31 samples of nematodes as follows: From intestines of Pimelodus clarias (L.)(27); Catfish (indeterminate)(3) and Pimelodella lateristriga (Müll. Trosch.)(1).