1. Stingless bees live in populous colonies that vary in size from a few hundred individuals to tens of thousands, although information on actual colony size is limited. Determining colony sizes using easily measurable biological parameters are important steps to understanding their life histories and ease their utilization and keeping. The objectives of this study were to determine the colony size of five Amazonian stingless bee species (Melipona flavolineata, Melipona fasciculata, Scaptotrigona aff. postica, Frieseomelitta longipes and Plebeia minima), and to identify biological parameters that covary with colony size. 2. The number of brood cells, adult bees, and food stocks were counted under laboratory conditions, alongside field assessments of egg-laying rate and external activity of adult workers. To identify covariates of colony size, the number of adult bees was regressed against the number of brood cells, egg-laying rate, external activity of adult workers, and food stocks, and the best Candidate models were ranked using the Akaike Information Criterion. 3. Mean (± s.d.) adult population sizes were: 1,046 ± 185 in M. flavolineata; 593 ± 300 in M. fasciculata; 7,404 ± 1,391 in S. aff. postica; 2,425 ± 1,000 in F. longipes; and 405 ± 254 in P. minima. We showed that the external activity is the biological parameter, after the number of brood cells, that presents the best relationship with the number of adult bees, which can be easily evaluated in the field.