In this paper, we investigate the data access patterns and file I/O behaviors of a production cosmology application that uses the adaptive mesh refinement (AMR) technique for its domain decomposition. This application was originally developed using Hierarchical Data Format (HDF version 4) I/O library and since HDF4 does not provide parallel I/O facilities, the global file I/O operations were carried out by one of the allocated processors. When the number of processors becomes large, the I/O performance of this design degrades significantly due to the high communication cost and sequential file access. In this work, we present two additional I/O implementations, using MPI-IO and parallel HDF version 5, and analyze their impacts to the I/O performance for this typical AMR application. Based on the I/O patterns discovered in this application, we also discuss the interaction between user level parallel I/O operations and different parallel file systems and point out the advantages and disadvantages. The performance results presented in this work are obtained from an SGI Origin2000 using XFS, an IBM SP using GPFS, and a Linux cluster using PVFS.