We consider one-parameter (C₀)-semigroups of operators in the space with infinitesimal generator of the form where G is an -valued rapidly decreasing distribution on ℝⁿ. It is proved that the Petrovskiĭ condition for forward evolution ensures not only the existence and uniqueness of the above semigroup but also its nice behaviour after restriction to whichever of the function spaces , , p ∈ [1,∞], , a ∈ ]0,∞[, or the spaces , q ∈ ]1,∞], of bounded distributions.