Ilmajoki is a municipality of Finland.
It is located in the province of Western Finland and is part of the Southern Ostrobothnia region. The municipality has a population of 11,720 (2003) and covers an area of 609.29 km2 of which 3.05 km2 is water. The population density is 19.2 inhabitants per km2.
The municipality is unilingually Finnish.
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