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To understand the infection strategies of the mosquito parasitic mermithid nematodes Romanomermis iyengari (Welch, 1964) and Strelkovimermis spiculatus (Poinar and Camino, 1986), we have investigated the foraging behaviors of the pre-parasites of both species. Romanomermis iyengari with body length of 0.99 ± 0.071 mm (twice as S. spiculatus 0.55 ± 0.01 mm) swims faster (2.76 ± 0.13 cm/min) than S. spiculatus (1.61 ± 0.09 cm/min). The nematode pre-parasites of R. iyengari showed negative phototaxis in responding to natural light, which caused them to migrate to shade where mosquito larvae tend to aggregate. This behavior helped the nematode to gain significantly higher parasitism when shade was present (37.7 ± 2.74%) than no shade (11.3 ± 0.95%). In contrast, S. spiculatus pre-parasites had no response to natural light and therefore, shade contributed no parasitism gain to this species. The host searching pattern data indicated that the two species used different strategies for host searching. The pre-parasites of R. iyengari tended to do long range search and responded to host aggregation habitat but slowed down and do more localized search when host present. However, the species of S. spiculatus , tended to do localized movement when hosts were absent regardless shade. Once host larvae present in nearby location, the nematodes responded to the host cue and moved faster toward their hosts. The active foraging behavior of R. iyengari may suggest that the species is more suitable for mosquito control in clear and moving water such as streams. However, the passive stay and wait behavior of S. spiculatus might be more suitable against mosquitoes in small sized water bodies such as temporarily flooded or stagnant ponds.