Explants of pith parenchyma excised from Romaine lettuce heads(Lactuca sativaLinn. var. Romàna) exhibited xylogenesis after four days dark incubation on a Murashige and Skoog (1962) medium containing auxin (IAA, NAA, or 2,4‐D), cytokinin (zeatin, kinetin, or benzyladenine), sucrose, and agar. With the exception of 2,4‐D, xylogenesis required both an exogenous auxir and cytokinin. The greatest numbers of tracheids were produced by the IAA (5.0 mg/liter)‐zeati (0.1 mg/liter) and 2,4‐D (0.07 mg/liter)‐zeatin (0.1 mg/liter) treatments, whereas the most effective treatment for callus formation was the NAA (0.5 mg/liter)‐zeatin (0.1 mg/liter) medium. In the absence of exogenous cytokinin, 2,4‐D stimulated xylogenesis after 14 days culture at a concentration of 0.02 mg/liter, but was ineffective at 0.07 mg/liter and 0.2 mg/liter. The 2,4‐D treatment induced tracheids to form in small meristematic nodules; aberrant tracheids were also observed. Different patterns in the differentiation of tracheids were associated with the various auxin‐eytokinin treatments. All IAA‐cytokinin treatments produced perpendicular strands around the periphery and tracheid formation throughout the lower half of the explants. The pattern of IAA‐induced xylogenesis was modified by the particular cytokinin employed. Treatment with NAA‐cytokinin induced horizontal strands which were branched, and the xylogenesis pattern was the same regardless of the cytokinin employed. The xylogenesis pattern produced by 2,4‐D‐cytokinin varied with the 2,4‐D concentration, and was independent of the cytokinin employed. Small numbers of tracheids were observed in explants cultured under xylogenic conditions and treated with caffeine (1000 mg/liter) for the inhibition of cytokinesis.