I try running (uiop/run-program:run-program '("where" "mvn.bat") :output :string) On asdf 3.1.7, it returns the correct output. With asdf 3.1.7.27 and 3.1.7.43. I get an error "Class not found: java.lang.UNIXProcess". If I go to the top level, and try again, I get the correct result. This is with: C:\Users\977315>java -jar "c:\mv-program-files\abcl-bin-1.4.0\abcl.jar" Armed Bear Common Lisp 1.4.0 Java 1.8.0_112 Oracle Corporation Java HotSpot(TM) 64-Bit Server VM Low-level initialization completed in 0.238 seconds. Startup completed in 3.973 seconds. Loading C:\Users\977315\.abclrc completed in 14.029 seconds. Type ":help" for a list of available commands. CL-USER(1): The abclrc is minimal: load asdf and load quicklisp: (eval-when (:compile-toplevel :load-toplevel :execute) (let ((file "asdf.3.1.7.43.lisp")) (load (merge-pathnames file (user-homedir-pathname))))) (eval-when (:compile-toplevel :load-toplevel :execute) #-quicklisp (let ((quicklisp-init (merge-pathnames "quicklisp/setup.lisp" (user-homedir-pathname)))) (if (probe-file quicklisp-init) (load quicklisp-init) (error "Did not find ~a ... quicklisp not loaded" "~/quicklisp/setup.lisp")))) I posted this on the ASDF mailing list, and Fare suggested this may be an ABCL-related issue. I hope all the problems I am having have one single root cause - and that we discover it :-)