Two hormone-based fertility-control treatments were trialed on free-ranging female koalas. Either levonorgestrel or oestradiol-17β was administered in a cylindrical, silastic, sub-dermal implant. Levonorgestrel was administered in a commercially produced implant (Norplant 2, Leiras). Two different doses of oestradiol were administered via custom-made implants of different length (1 cm and 0.5 cm). Treatments were randomly applied to 58 females (each koala receiving a single implant) and a control group of 27 female koalas received no implant. Fertility, as determined by the presence of pouch young, was recorded following capture and examination during June 1998, 1999, 2000 and 2001. Fertility was reduced in all treatment groups but remained high (90%) in the untreated group. Fertility was lowest in koalas that received levonorgestrel (0%) and longer oestradiol implants (5%). The results demonstrate that slow-release implants containing either of these two steroid hormones have the ability to significantly lower fertility of wild koalas for at least four breeding seasons following treatment. No adverse side effects were apparent in any of the treated individuals. Compared with the cost of current management strategies for over-abundant koala populations, their deployment on a large scale should be cost-effective.