The effectiveness of long-lasting preimpregnated nets of Permanet type (deltamethrin, 50 mg/m2) erected in households in rural areas of Côte d'Ivoire was tested on two laboratory strains of Anopheles gambiae s.s.: the Kisumu susceptible strain and the Vk per pyrethroids resistant strain with >70% kdr allelic frequency. Treated nets were distributed in households in three villages of Danan6 forest area in western part of Côte d'Ivoire. In each village, a net was sampled for bioassays. Three Permanets also were erected in the laboratory, serving as control samples. From May 2001 to July 2002, the effectiveness of these deltamethrin-pretreated nets was monitored using World Health Organization-cone tests on the two strains of An. gambiae. Mortality rates were recorded 24 h postexposure. Knockdown times for 50 and 95% mosquitoes (kdT50 and kdT95, respectively) were estimated by means of WIN DL software. One-way analysis of variance was used to compare the knockdown times. Times to failure of nets were analyzed using Cox model. The kdT50 of the Kisumu susceptible strain with both laboratory samples and nets used in the field varied around 10 min. No significant difference was recorded between the kdT50 of the Kisumu susceptible strain with laboratory kept nets and samples of nets used in the field. The kdT95 values were in the same scale with the two types of nets. The kdT50 of the Vk per resistant strain when exposed to used nets were twofold that of the Kisumu susceptible strain at the beginning of the trial, and they increased to fivefold 15 mo later. These latter kdT50 significantly differed to those of the Kisumu susceptible strain tested with laboratory and field samples of nets. The kdT95 significantly differed from those of the Kisumu strain with laboratory kept nets and with field kept nets. Baseline bioassay mortality rates were always 99-100% with the Kisumu susceptible strain, and they did not show any significant difference between laboratory-kept nets and field-used nets during the 15-mo trial. With the Vk per resistant strain, the expected long-lasting activity was not achieved. A high decrease of mortality rates was observed from 69 to 75% in the first 3 mo to 2% at the month 15. This mortality was associated with significant differences between Vk per resistant strain tested with field-used nets compared with Kisumu susceptible strain tested with both laboratory kept-nets and field-used nets. This study emphasized the actual long-lasting effectiveness of Permanet against the An. gambiae Kisumu susceptible reference strain and a rapid decrease of residual activity against a strain with kdr-based resistance to pyrethroids.