The weight of the seeds of the cotton or "seed-index" (SI) it varies according to the cultivation conditions and the varieties. The SI it influences on the fiber percentage (%F) in the desmotadoras, besides their importance in connection with the germinative aptitude. The variations, in relation to the variety and the cultivation region, of SI and %F, they are studied with the data of the rehearsals of varieties carried out in different towns of the area of cultivation of the cotton in Paraguay (it was discarded rehearsals with SI inferior to 8 g, associated to fiber percentages also very low and indicating abnormal agronomic conditions). According to the town, the average plurianual of SI it varies of until + / - 0,7 g; for %F the variation is until + / - 2,5 points. The data show a negative relationship among SI and %F: heavier the seed, minor is the %F to the desmote. In consequence, the towns that donate heavier seeds usually give less fiber, but the correlation doesn't appear very strict: the east of the oriental region gives an SI bigger with inferior %F, but the north gives an SI a little inferior with %F also inferior. The data characterize the studied varieties well; Guazuncho 2 and Reba P279 shows an important effect of the variations of the SI on the %F (slopes -0,8 at -1,0), while with IAN 338 and CD 401 the slopes are smaller and he/she gets lost less in %F when the SI it increases.