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Previously, we found a significant negative correlation between DNA C-value and altitude among eight natural populations of Dactylis glomerata L. (Creber et al., New Phytologist 128: 555–561, 1994). We have examined the extent to which similar negative relationships exist in other altitudinal transects, one in southern France and the other in Italy. Using Feulgen microdensitometry, C-values were negatively correlated with altitude both for the French and Italian populations. A combined plot of DNA C-values against altitude for all of the transects (representing C-values for 17 natural populations), exhibited a highly significant negative relationship; there was a 1·3-fold variation in DNA C-value from the largest genome-lowest altitude to the smallest genome-highest altitude natural population. Such a consistent marked altitudinal cline suggests strong nucleotypic selection acting upon these populations with increasing altitude. Preliminary examination of amplified fragment length polymorphisms between populations selected from the upper and lower limits of the French and Italian transects has shown that these populations are genetically distinct. The extent to which this genetic separation is related to altitude or genome size, or both, is discussed. [ABSTRACT FROM AUTHOR]