The present research of larval breeding sites was performed. The larvae of 14-species of mosquito comprising 3-genera viz. Anopheles, Culex and Aedes were collected from different ground water habitats in Karachi and Thatta of Sindh province, Pakistan, during four years period from 2004 to 2007. A total of 94800 mosquito larvae were collected, out of all these larvae 13.82% was belonged to genus Anopheles, 81.03% to genus Culex and only 4.77% belonged to genus Aedes. Over all collection during 2004 to 2007, the Anopheles was found 16.00%, 14.11%, 11.93% and 13.38% respectively. While Culex larvae were found 79.90%, 82.41%, 80.84% and 82.31respectively during 2004 to 2007, whereas Aedes larval population was recorded as 4.09%, 3.48%, 7.23% and 4.31% during four years. Among Anopheles six species were found which are An. annularis, An.culicifacies, An. pulcherrimus, An. nigerrimus, An. stephensi and An. subpictus (12,974 larvae were studied and identified). Similarly six species of Culex genus (77,192 larvae were studied and indentified) as Cx.xx bitaeniorhynchus, Cx. fuscocephalus, Cx. pipiens fatigans, Cx. pseudovishnui, Cx. tritaeniorhynchus and Cx.vagans. In the same way two species of Aedes, aegypti and Aedes albopictus were also found. Culex genus was the most common and showed highest percentage among Culex species, the Cx. tritaeniorhynchus and Cx. pipiens fatigans exhibited the greatest range of habitat. Among Anopheles, An. culicufacies and An. stephensi breeds in almost any available type of water, but An. stephensi was inversely related to temperature as for as Aedes mosquito concerned, Ae. aegypti percentage was always very high as compared to Aedes albopictus. Highest number mostly was found in the month of October, every year. It is to be noted that larvae fauna depends not only the type of habitat but also on the physical and chemical composition. Some species have positive association and some restricted only in clean water. In the present thesis different experiments were conducted for the resistance on mosquito larvae of District Karachi and Thatta, Mosquito larvae were exposed to neem formulation, the Biosal (10 EC) available locally in the market, a synthetic pyrethroid, deltamethrin (10 EC) andxxi Acorus calamus extract were used. The LC 50 values were calculated simultaneously the two enzyme (GOT, GPT) were estimated in mosquito larvae. Residue analysis and rate of biogradation was also noted by HPLC The LC 50 values of Biosal deltamethrin and Acorus calamus extract against mosquito larvae were computed as 1605.05 ppm, 0.6119 ppm and 70.64 ppm respectively. The enzyme activity pattern and inhibition by Biosal, deltamethrin and Acorus calamus extract were estimated after treatment with LC 50 values of the under test insecticides. The inhibition of GOT produced by biosal, deltamethrin and Acorus calamus extract was calculated as 5.78%, 26.66% and 5.83% respectively. Whereas for GPT the inhibition was 3.95%, 19.65% and 13.77% respectively. The residue analysis by HPLC shows that 97.79%, 87.87% and 78.18% residue of biosal were detected in 24, 48, and 72 hours exposed samples as compared to standard biosal sample. While 98.66 %, 98.49% and 70.45% residues of deltamethrin were detected in 24 hours, 48 hours and 72 hours exposed samples. Whereas 94.63%, 71.65% and 66.25 % residue of Acorus calamus extract were found in 24 hours, 48 hours and 72 hoursxxii exposed larvae. The residue analysis by HPLC indicated that the biodegradation is faster in biosal as compared to Deltamethrin and Acorus calamus samples, which indicate that Biosal degrade quicker and this may be concluded that toxicity of pesticide if compared to each other, is in the following sequence Biosal > Acorus calamus > Deltamethrin. Enzyme inhibition values and HPLC residue analysis, possibly proves the hypothesis that the pesticides obtained from neem and other plant and trees are not only less harmful but safer for our environment, but less prone to the development of resistance.