The main types of S. mansoni eggs observed in faeces of patients coming from endemíc areas and never before submitted to treatment are described. The finding of mature and immature viable eggs, as well as of several types of dead eggs, including the calcified (black eggs), is demonstrated. Attention is drawn to the rarity of the last mentioned type of eggs, which, however, must not be overseen in the coproscopic control after anti-schistosomal therapy. The main dimensions of the eggs observed as well as their photomicrographs are presented. A counting of S. mansoni eggs per cm3 of faeces was made. Higher counts of viable as well as dead eggs were found in the hepato-splenic form of the disease than in the hepato-intestinal form. However, it was observed that the percentage of the several types of eggs didn't differ very much in the two forms. It was supposed that besides other facters a higher number of worms in the host shouli play an important part in the development of the severe forms of schistosomiasis mansoni. Subjects under 20 years had higher egg counts than those over 20. However, the percentage of the several types of eggs showed no great difference between the two age groups of patients. It was believed that besides the immunological factors the aging of the worm during the parasitaemia can be responsible for the decrease of the oviposition. The data related to the global counting of S. mansoni eggs in faeces of patients of non endemic areas confirmed, in general, the results obtained by other authors in S. mansoni endemic areas, regarding the higher or lower egg counts in relation to the clinical form of the disease or the age of patients.