A survey of citrus orchards of northern Iran revealed that the citrus nematode, Tylenchulus semipenetrans, was present in 89% of soil and root samples collected. The number of juveniles and females averaged 6490/250 ml soil and 214/g root, respectively, in infested samples, and the mean numbers of juveniles and females in soil and root samples of sweet orange and sour orange rootstocks were greater than the damage threshold level in most regions. Population patterns of citrus nematode on sour orange Citrus aurantium and Poncirus, Poncirus trifoliata, rootstocks were monitored for 2 successive years. Nematode population densities peaked once a year during July-August and declined to lower levels during autumn and winter on both rootstocks. The duration of the life cycle of T. semipenetrans was 8 and 9 weeks on sour orange C. aurantium and Troyer citrange, C. sinensis × P. trifoliata, seedlings, respectively, at 24 ± 2°C. In glasshouse experiments, T. semipenetrans was controlled on sour orange seedlings by the granular nematicides, fenamiphos and cadusafos, the doses 4 and 8 ppm having the greatest effect when applied simultaneously with nematodes, whereas the nematicides Vydate and fenamiphos at 8 ppm showed greatest effects on nematodes in roots when applied 6 weeks after nematodes had been added to the soil. The effects of granular fenamiphos and cadusafos (10G) on T. semipenetrans population densities were also investigated in infested citrus orchard with rates 1.0 and 2.0 g a. s. of nematicides/m2 for 1 year. Although the nematicide treatments reduced nematode population densities in soil and roots, fruit weight and size was not affected by these treatments.