Seasonal drought in tropical moist forest may be the cue for fine root death and turnover, and it may signal root growth deeper to access subsurface water and (or) nutrients. We examined these predictions by measuring fine root biomass (<2 mm diam.) and the timing of root growth and disappearance in an old growth tropical moist forest on Barro Colorado Island (BCI), Republic of Panama, in the fifth year of a dry season irrigation experiment. Irrigated soil had greater available P concentrations; however, a more pronounced effect was less stable soil aggregates causing higher bulk density. Irrigation did not affect fine root biomass. Mean (±SE) biomass between 0 and 30 cm was 372 ± 63 g/m2 within control versus 286 ± 39 g/m2 within irrigated plots. Mean biomass between 45 and 75 cm was 74 ± 7 g/m2 within control versus 62 ± 7 g/m2 within irrigated plots. Dead roots were less than eight percent of the total. We characterized root growth using in-growth screens (1.7 mm mesh) installed between 0 and 15 cm. Root density in the screens peaked soon after the rains began in the control plots but during the dry season in the irrigated plots. Very few dead roots accumulated in the screens, with no differences seasonally or among treatments. We developed a model to estimate birth and death rates of fine roots using root densities in the in-growth screens and the disappearance of roots laced into screens and incubated in situ. Inferred root birth rates were greatest in the early part of the wet season in control plots and in the dry season in irrigated plots. Inferred mortality rates of fine roots less than four months old were reduced by irrigation, but this was offset by much greater disappearance of six- to eight-month-old roots in the irrigated plots. Although irrigation altered the timing of root growth and mortality, roots did not grow year-round in the always-wet soil. Therefore, soil water availability was an important cue for root growth, but an inherent seasonal cycle remained. Fine roots died continuously throughout the year and disappeared quickly.