The biology of Euastacus mirangudjin, a small freshwater crayfish from subtropical eastern Australia, is described for the first time. Long-term monitoring was undertaken at a site in Toonumbar National Park to gather data on growth, moulting, maturity and reproductive activity. To complement field observations, egg and juvenile development was further studied in laboratory aquaria. The orangebellied crayfish reaches less than 40 mm carapace length, with female maturity occurring near 30 mm. Animals do not respond to baits, and appear to spend most of their time within their burrows. The species is an Autumn-Spring brooder with a distinctly low fecundity and large, ovoid eggs (3.0 x 3.8 mm), and females do not appear to routinely moult prior to spawning. At some sites, the species co-occurs with the much larger and spinier E. sulcatus. A high incidence of regenerate chelipeds, missing limbs and other wounds was recorded. Euastacus mirangudjin hosts a small, unpigmented Temnocephalan flatworm. The biology and ecology of the species is discussed with reference to other species in the genus Euastacus.