Low agricultural commercialisation due to low productivity and a lack of access to and use of improved seeds are common features of smallholders in the Ethiopian highlands. Seed-producer cooperatives (SPCs) were established and strengthened in these highlands to facilitate smallholders’ access to improved seed. Using survey data collected from 425 randomly selected smallholders, a full-information maximum likelihood endogenous switching regression (ESR) model and a propensity score-matching (PSM) technique were employed to estimate the impact of membership of seedproducer cooperatives on the output and input commercialisation of members and non-members. The results show that farm size, availability of extension services, household size, farm income, participation in other farmers’ organisations, access to training, and education of the household head are key factors associated with cooperative membership. The results from the ESR and PSM models are consistent and show that being a member had a positive and statistically significant impact on tef and wheat farmers’ output and input commercialisation. Non-members’ status in output and input commercialisation would have been improved if they had been members of seed-producer cooperatives.