A programme to monitorHelicoverpa armigera(Hübner) for resistance to fenvalerate was initiated in 1991. Bioassays that exposed individuals to insecticide residues in glass vials were carried out on pheromone‐trapped adults and on Fl larvae reared from insects collected in the field. These assays assessed changes in susceptibility to fenvalerate during each cropping season and from year to year.H. armigerafor assays were collected from processing tomato, sweet corn, and lucerne crops, mainly from the Gisborne and Hawke's Bay regions of New Zealand. Comparison of LD50S showed significant increases in resistance of 69‐fold for adults and 47‐fold for third instar larvae within a season. A diagnostic dose of 1 μg (LD95for susceptible populations) showed a significant trend of declining mortality from 1992 to 1994 for adults, but trends were unclear for larvae. Assays with a diagnostic dose of 10 μg (LD99for susceptible populations) showed decreasing mortality from 1992 to 1994, and at 90 μg c. 1% survival of field‐collected adults was detected in 1994. Comparison with Australian data and lack of control failure suggests that New Zealand populations are still widely susceptible to the insecticide, but that the frequency of resistance to pyrethroids is increasing. A reduction in the rate of decrease in susceptibility within the 1994 cropping season suggests that integrated pest management procedures have reduced selection for fenvalerate resistance inH. armigera.