The present work evaluated the number of Rondonia rondoni in intestine of Piaractus mesopotamicus, for difference of dry and wet weight of parasites samples by each host, from the relation dry weight and number of parasites. Totals samples of R. rondoni of 37 specimens of P. mesopotamicus for attaiment of wet weight, dehydrated in stove with temperature between 55 degrees C and 60 degrees C, the wet weight was measured after 24 h. The number of parasites was calculated with rule of three, considering the average number of 1,010 +/- 6.027 specimens quantified in previous assay. The equation of linear regression is y = 13.138x - 162.01 and r(2) = 0.9989. The normality of the data was proven with Kolmogorov-Smirnov (p < 0.01).