This paper describes MENDELS ZONE, a Petri-net-based concurrent programming environment, which is especially suitable for cooperating discrete event systems. MENDELS ZONE adopts MENDEL net, which is a type of high level (hierarchical colored) Petri net. One of the characteristics of the MENDEL nets is a process-oriented hierarchy like CCS, which is different from the subnet-oriented hierarchy in the Jensen's hierarchical colored Petri net. In a process-oriented hierarchy, a hierarchical unit is a process, which is more natural for cooperating and decentralized discrete event control systems. This paper also proposes a design methodology for MENDEL nets. Although many Petri net tools have been proposed, most tools support only drawing, simulation, and analysis of Petri nets; few tools support the design methodology for Petri nets. While Petri nets are good final design documents easy to understand, analyzable, and executable it is often difficult to write Petri nets directly in an earlier design phase when the system structure is obscure. A proposed design methodology makes a designer to construct MENDEL nets systematically using causality matrices and temporal logic. Furthemore, constructed MENDEL nets can be automatically compiled into a concurrent programming language and executed on a parallel computer.