<i>Trifolium subterraneum</i> L. is widely grown in the phosphorus (P) deficient soils of southern Australia. However, this pasture legume has a high critical external P requirement and requires frequent applications of P fertiliser to achieve high productivity. Twenty-six genotypes of <i>T. subterraneum</i> were grown to determine: (<i>i</i>) differences in shoot growth and P acquisition under low-P supply; (<i>ii</i>) the root morphological traits important for P acquisition; and (<i>iii</i>) the feasibility of selection among genotypes for these root morphological traits. Micro-swards of each genotype were grown with a topsoil layer that was either moderately P-deficient or had P supplied in excess of the critical requirement for maximum yield; the subsoil layer was P-deficient. Yield and P content of shoots and roots were determined after 5 weeks’ growth, and root samples were assessed for diameter, length and root hair length. All genotypes were equally highly productive when excess P was supplied. However, relative shoot yield in the moderately P-deficient soil ranged from 38-71%. Total root length ranged from 63-129 m pot^-1, and was correlated with total plant P uptake (R² = 0.78, <i>P</i> < 0.001). Variation was also observed in average root diameter (0.29-0.36 mm) and root hair length (0.19-0.33 mm). These traits were combined with root length to calculate the total surface area of the root hair cylinder, which was also correlated with total plant P uptake (R² = 0.69, <i>P</i> < 0.001). The results demonstrated that there was significant variation in P acquisition efficiency and shoot yield among genotypes of <i>T. subterraneum</i> when grown in P-deficient soil, and that root length was important for improved P uptake. The results indicate potential to identify superior genotypes that achieve improved P acquisition and higher shoot yields in low-P soil.