Piggery slurry was aerobically treated in a farm scale treatment plant with mean residence times of between 1·7 and 6·3 d. The treated slurry was analysed using standard laboratory methods including key parameters such as COD (chemical oxygen demand) and VFA (volatile fatty acids) concentrations. Further samples were taken and analysed either fresh from treatment or after a period of storage, for odour using olfactometric methods. These included the determination of odour concentration, by dynamic dilution, and offensiveness and intensity, by use of a panel score.Chemical analysis of the treated samples indicated a breakdown of organic material broadly in line with that expected, although, in the short treatment, insufficient aeration may have retarded the extent of the process. The olfactometry clearly demonstrated that reduction of odour in terms of concentration and offensiveness were achieved by aerobic treatment. The reduced level of odour was evident even after 28 d of subsequent anaerobic storage. Typically, the treatment reduced the concentration by 50–75% although this was insufficient to reduce the perceived intensity at source to below two, (equating to “faint odour”) from the high valve of over five (equating to “very strong odour”) measured for the untreated slurry.The effect of the duration of treatment on odour abatement was mixed. Reductions of odour concentration were broadly similar in all treatments. However, in terms of odour offensiveness, the best result was clearly achieved by the longest treatment of 6·3 d. In this case, the score of the odour quality of the freshly treated slurry was below two (equating to better than “faintly offensive odour”), compared with a score of four for the untreated slurry (equating to “strongly offensive odour”)