This study examines the developmental basis of morphological variation in the lichen Ramalina menziesii Tayl., whose thallus varies from highly reticulate forms typical of inland populations to fine dissected forms characteristic of coastal habitats. Development was studied microphotographically at two coastal field sites and compared with previous studies carried out at an inland site. In all three populations nets developed from perforate buds, enlarged by diffuse expansion, and produced new reticulate tissue from the inrolled apical margin. However, nets of coastal thalli showed greater degrees of perforation and expansion within the young tissue of the apex. Frequently, damage to these weaker apices disrupted the production of new reticulations at an early stage of net growth. Subsequent expansion of disrupted reticulations and proliferation of daughter nets at the broken ends appeared to accentuate gross morphological differences. Two types of deviation from the basic pattern of bud development were also seen occasionally in all populations examined.