We study the strict type assignment system, a restriction of the intersection type discipline \cite {Barendregt-et.al'}, and prove that it has the principal type property. We define, for a term $M$, the principal pair (of basis and type). We specify three operations on pairs, and prove that all pairs deducible for $M$ can be obtained from the principal one by these operations, and that these map deducible pairs to deducible pairs. Appeared as: @Article {Bakel-JLC'93, Author = "S. van Bakel", Title = "Principal type schemes for the {S}trict {T}ype {A}ssignment {S}ystem", Journal = "Logic and Computation", Volume = "3", Number = "6", Year = "1993", Pages = "643-670" }