This volume contains the proceedings of the 16th International Symposium on Mathematical Foundations of Computer Science, MFCS '91, held in Kazimierz Dolny, Poland, September 9-13, 1991. The series of MFCS symposia, organized alternately in Poland and Czechoslovakia since 1972, has a long and well established tradition. The purpose of the series is to encourage high-quality research in all branches of theoretical computer science and to bring together specialists working actively in the area. Principal areas of interest in this symposium include: software specification and development, parallel and distributed computing, logic and semantics of programs, algorithms, automata and formal languages, complexity and computability theory, and others. The volume contains 5 invited papers by distinguished scientists and 38 contributions selected from a total of 109 submitted papers.