Third instar larvae of the ant Myrmica rubra were treated with the antibiotics streptomycin or chloramphenicol. Antibiotics increased the growth rate of small (<1 mg) larvae compared with controls of the same weight, and decreased the growth rate of large (>2.0 mg) larvae compared with paired controls. Assuming that the antibiotics reduce the number of bacteria in the larval gut, our results are consistent with the hypothesis that the weight of 3(rd) instar larvae is at least partially determined by the 'bacterial load' in their guts. A heavy bacterial load suppresses larval growth and results in small larvae, whereas large larvae have lighter loads. Because the larval developmental pathway (worker-determined or queen-potential) is related to size, the results are discussed with respect to caste determination in M. Rubra. Antibiotic treatment was also observed to break diapause, an interruption in growth that large 3(rd) instar queen-potential larvae normally experience and a growth hiatus thought to be an essential step towards queen development. Despite this interruption in diapause, larvae did develop into queens and a significantly greater proportion of queen pupae were produced during the antibiotic interrupted diapause period than after. It was concluded that diapause in this species may be affected by bacterial load and that bacterial load may be a significant factor in caste determination during diapause.