The inhibitory effect that suckling has on the reproductive function of primate mothers varies as a function of the intensity with which they are suckled. Here we present an easily computed index of one parameter of suckling intensity, namely the temporal patterning of suckling bouts. High intensity suckling is characterized by frequent nursing bouts demarcated by short interbout intervals. Therefore, our suckling index is based on the brevity of observed interbout intervals, more specifically the proportion of such intervals that fail to exceed a criterion length. The index is an appropriate means of making interspecific comparisons of the development of infant suckling and is well suited for application to field data that include interbout intervals that were not observed in their entirety. To demonstrate its utility, we apply the index to field data collected on the suckling behavior of free-ranging rhesus monkey (Macaca mulatta) infants in India. In this context, we demonstrate that, in rhesus, between-infant differences in suckling intensity manifest themselves early in the postpartum period and contribute to between-female differences in the timing of first mating postpartum.