We designed and field tested an expandable, easy-to-build radiocollar for subadult male fallow deer (Dama dama). This collar is inexpensive (it only costs few euros), and breaks away due to the natural growth of the neck without causing injury. Elastic rubber bands (expandable section) are covered by a leather layer that protects it from premature breakaway. We evaluated the effectiveness of 37 expandable collars deployed on pricket (1-2 y.o.) fallow deer, by using the Kaplan-Meier survival analysis. Average duration of collars from their deployment to their natural drop off was almost 2 years (mean±SE, 91.7 ± 6.4 weeks). This enabled us to monitor subadult fallow deer during maturation and to collect data on their spatial behaviour for the first time. Very little is known on the behavioural ecology and life history of subadult cervids. Accordingly, breakaway radiocollars are important research tools whose use should increase among field biologists.