Pearl millet (Pennisetum glaucum (L.) R. Br.) is commonly grown at a wide range of plant population densities, both by design and as a consequence of stand establishment problems. High tillering genotypes are known to compensate effectively for lower plant population densities through their tillering capacity; less is known about the ability of naturally low tillering genotypes to adjust to low plant population densities. This is a particular concern in the case of the Iniadi landrace materials which are currently widely used in breeding programmes in both India and Africa. This research was done to determine how effectively the low tillering Iniadi types adjust to low plant population densities and how their mechanism(s) of adjustment compares to those of higher tillering materials. Two high and two low tillering genotypes were grown over a period of 5 years at plant population densities ranging from 12 to 2 plants/m2, under both high and low fertility regimes at the ICRISAT Centre, India. Both the high and low tillering types adjusted equally well to the reduced plant population densities, as judged by grain yield, but differed in their mechanism of adjustment. The high tillering genotypes adjusted, as expected, primarily by increasing productive tiller numbers, with only small changes in individual tiller productivity. The Iniadi genotypes increased productive tiller numbers in response to decreasing plant population densities to a limited degree, but increased panicle productivity to a much greater degree than the high tillering types. There was no differential effect on adjustment ability between the two types as a consequence of increased fertility, despite the stimulating effect of fertility on productive tiller numbers. The results are discussed in terms of generalized mechanisms of response to changing individual plant environmental resources (fertility and space), and in terms of the use of Iniadi germplasm in pearl millet breeding programmes.