A defect tolerant approach to fatigue modelling for constant amplitude loading was developed by Bruzzi and McHugh (Int. J. Fatigue 24 (2002) 1071). It was applied to two metal matrix composites (MMCs): (1) a forged 2124 Al reinforced with 17% SiC particles and (2) a cast 359 Al reinforced with 20% SiC particles MMC, in Bruzzi and McHugh (Int. J. Fatigue 25 (2003) 577). However, no explicit representation of the two-phased microstructure was considered. The material was assumed to be homogeneous and behave as a continuum. In this work, the extreme effects of the reinforcing particles on the growth behaviour of a small fatigue crack, using the modelling methodology of Bruzzi and McHugh, is investigated for the Al 2124 MMC. This is performed using micromechanical models, in which separate representations of the matrix and the reinforcing particles are included, in order to assess local stress fields within the matrix. Two extreme cases of idealised crack growth are examined: (1) a predefined crack path approaching a reinforcing particle and (2) a predefined crack path lying between two reinforcing particles. The predicted stress gradients along each of the predefined paths are then included in the fatigue modelling methodology described by Bruzzi and McHugh. Finally, the predicted results of the extreme cases of the short crack growth behaviour for the two-phase Al 2124 MMC are presented, discussed and compared to the short crack growth rate behaviour of the homogenised MMC model of Bruzzi and McHugh. (C) 2004 Elsevier Ltd. All rights reserved.