Growth modelling in three Spanish chicken genetics types and a COMMERCIAL line SASSO. Effect of the type of housing The growth rate of chickens from the Castellana Negra (CN), two genetics types of indigenous Catalonian breeds improved in the Unit of Genetic Poultry from IRTA in Mas Bové, the Penedesenca Negra (PN) and the Empordanesa Roja (ER), and a commercial semi-heavy SASSO line (SS) were compared by means of the Gompertz-Laird growth curve. Experiment was conducted in the province of Soria (North Central Spain). Birds were reared either in captivity or in free-range systems. It was controlled the individual weight of 90 chickens of each genetic type until 14 life weeks. Castellana Negra showed a lower initial growth rate than the other 3 genotypes (0.579 wk-1 vs 0.737-0.805 wk-1) and reached a lower asymptotic weight (2583 g vs 3195-4050 g). Decrease of growth rate after inflection was greater in PN (0.184 wk-1), lower in CN (0.140 wk-1) and intermediate in SS and ER (0.163 wk-1 and 0.172 wk-1). Maximum growth rate was reached at a lower age in PN and ER birds (8.0 wk and 8.7 wk) than in SS and CN birds (9.2 wk and 10.1 wk). Weight at maturity was higher for birds reared in captivity in PN and SS (+267 g and +180 g), for birds reared in free-range in CN and ER (+140 g and +47 g).