Montgomery County health officials made the decision after more mosquito pools tested positive for West Nile Virus.
32 more pools were positive - 17 in Norristown, 9 in Upper Merion, 3 in Lower Merion, and 1 each in Conshohocken, Worcester, and Pottstown.
The majority of the positive samples were collected before last week's spraying, however, health officials decided to spray to complete the eradication.
Weather permitting, the spraying will begin at dusk tomorrow in Worcester Township. that is when mosquitoes are most active.
Teams will spray in Pottstown Borough and Upper Merion Township on Thursday night.
It will be done by ground units only - no aerial spraying.
This year, the county has had 59 mosquito pools test positive for West Nile virus, four times as many as in 2007.
There has been one human case of ac tive West Nile Virus reported to the Health Department. It came in a resident from Lower Merion Township.