The Colombian case offers a rare opportunity to observe effects of electoral reform where districting remains constant. Only the formula changed, from extremely 'personalized' (seats allocated solely on candidate votes) to 'listized': seats are allocated to party lists, which may be either open or closed. Electoral reform has effects on both the inter-party dimension (the number of parties competing) and the intra-party dimension (the extent of competition within parties). Consistent with theoretical expectations, the inter-party dimension features an increased number of parties in the low-magnitude districts and a decrease in the high-magnitude districts. On the intra-party dimension, the impact " mirrors" the inter-party: less competition in smaller districts, yet more in larger districts. © 2010 Elsevier Ltd.