A series of compounds containing either non-proteinogenic β-/γ-amino acids or N-substituted β-alanine residues (β-peptoid units) in P1 specificity position were synthesized based on the structure of sunflower trypsin inhibitor 1 (SFTI-1). The compounds were synthesized on a solid support; the N-substituted β-alanines (βNhlys and βNhphe) were introduced into a peptidomimetic chain via a two-step approach using acryloyl chloride and an appropriate primary amine. The inhibitory activities were characterized in vitro against bovine α-chymotrypsin or bovine β-trypsin. Three analogues displayed activity comparable to fully proteinogenic counterparts—monocyclic SFTI-1 and [Phe5]SFTI-1. Moreover, all active peptidomimetics were resistant toward proteolytic degradation, even after 24-h incubation at room temperature. © 2012 Wiley Periodicals, Inc. Biopolymers (Pept Sci) 100: 154–159, 2013.