Distribution of the citrus nematode (Tylenchulus semipenetrans) was studied over 18 months in a 6-year-old orchard of grapefruit (Citrus paradisi cv. Ruby Red) on sour orange (C. aurantium) rootstock. The 1.8-ha orchard was under chemical weed control, no tillage, and flood irrigation. Highest numbers of nematodes were found in the top 15 cm of the soil profile. The nematode population peaked in April and declined to lowest levels in August and September. Numbers of nematodes were negatively correlated (r = -0.95) with soil temperatures above 29 C. Soil populations of nematodes were not correlated with soil moisture. The distribution of the nematode in the field was highly skewed and was described by a negative binomial. In this 1.8-ha block, five soil samples of 12 cores each would provide an estimate within 20% of the true nematode population mean with 95% confidence.