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Pea plants exhibit density-dependent habitat selection as they grow.We split the root of a young pea (Pisum sativum L.) so that half grew in one pot and half in an adjacent pot. The rest of the plant remained intact. This is a 'fence-sitter plant'. Each root-half was exposed either to no competition in its pot or to competitor plants sharingits pot. There were one, two, three or five competitor plants. The total root biomass and the fitness (= dry weight of fruit) of the fence-sitter decreased only slightly and insignificantly in response to increased density of the competitor plants. The fitness of the competing plants decreased with density. The fence-sitter shifted its root system from the pot with competition to that free of competition in proportion to the number of competitors. The fence-sitter apparently invested in each of its two roots so that the ratio between the roots was similar to the ratio between the resources in the pots. This result is analogous to the habitat-matching rule of the ideal free distribution of populations (Fretwell, 1972). We suggest that plants invest in each of their roots until the uptake rate per unit root biomass isequal for all roots [ABSTRACT FROM AUTHOR]