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Embryonic development in animals is dynamically regulated by physiological, behavioural, and environmental factors (temperature, precipitation, humidity), which in turn influence the timing of birth or hatching. In the present study, we provide evidence that parturition in a large-bodied North American pitviper, the western diamond-backed rattlesnake (Crotalus atrox), is environmentally cued. Specifically, we tested the hypothesis of births coinciding with rainfall events during the second-half of the monsoon season (late July to mid September) using randomization modelling. Twenty-one adult females surgically implanted with radio-transmitters were tracked for extended periods from 2001-2010. From 2003 to 2007, the 21 females gave birth to 38 litters, generating sufficient data to test our hypothesis. In all years, births were restricted to a 4-week period from 5 August to 7 September, which spanned between 6 and 19 days (mean +/- SD, 15 +/- 5.2 days). Most births (92.1%) occurred in August. Births were significantly associated with rainfall events in 2007, although births in 2003 and 2005 occurred closer to rain events than randomly generated births for respective years. However, when birth events across all 5 years were pooled, the model indicated a significance difference in mean rain-days versus random rain-days. Hence, births occurred more closely to rain events than random days. Other variables associated with monsoon events (increases in cloud cover and humidity; changes in barometric pressure) were not measured but constitute potential cues. The present research is the first long-term, individual-based radio-telemetric study of a snake species to investigate environmental cues related to parturition using procedures of randomization modelling.(c) 2013 The Linnean Society of London.