Milicia excelsa (Iroko) is a valuable timber species occurring naturally in the humid forests of Africa. The high quality and great demand of wood obtained from M. excelsa has resulted in its over-exploitation. Attempts at establishing large scale plantations to ensure sustained production have been hampered by the gall forming psyllid Phytolyma lata. In spite of the economic importance of P. lata little research has been conducted on the impact of the psyllid on the establishment of M. excelsa plantations. The effect of P. lata on growth, dry matter accumulation and allocation of M. excelsa seedlings is assessed in this study. Seedling growth in infested plots was significantly reduced. Plants infested had lower height, stem diameter and biomass growth. Mean yield losses of stem, branches, and leaves of infested plants were 68.9%, 48.3% and 64% respectively of matched uninfested plants. Infested plants also had smaller, fewer and highly chlorotic leaves. There were no significant differences (P > 0.05) in yield, specific leaf area, leaf area ratio and leaf weight ratio of plants growing in the open screen house and open plot without screening (infested plots) even though irradiance incident in the open screen house was 70% of that in the open plot without screening. It is concluded that P. lata infestation has significant effect on growth and biomass accumulation in the early development of Iroko and that such information would improve integrated pest management (IPM) programme for P. lata. Secondly, biomass allocation of M. excelsa seedlings is not significantly different probably at irradiances greater than 70% ambient full sun.