Abstract. A thematic map displays the spatial patterns of one or more data themes for a specified geographic area. Thematic maps are used, for example, in geospatial data distribution and exchange, multi-source geospatial data aggregation, visual communication, and decision support. The latest web technologies and open geospatial standards make it feasible to produce on-demand thematic maps in an internet environment. This article describes a geospatial web service approach for creating customized thematic maps of cropland data layer (CDL) data. The design and implementation of the geospatial web service using open-source software are described in detail. The service meets the requirements of producing, sharing, and printing CDL thematic maps for the areas of interest without requiring additional mapping software. Examples of service invocation are given to demonstrate the applicability of the proposed approach. This service offers a new dissemination channel to distribute CDL data to the general public. Feedback from users in the agricultural community shows that the implemented web service has greatly improved the availability, customization, and timeliness of CDL thematic maps, reduced operational resource requirements and cost, facilitated geospatial data distribution and usability, and broadened the reach of geospatial cropland data products. In the first 20 months of availability, users have produced over 10,000 thematic maps.