Abstract: A grass catching attachment for use with a rotary lawn mower is disclosed. The attachment includes a flexible bag which has been adapted to expand the capacity of a rigid, grass catching receptacle frequently employed by commercial landscapers during mowing operations. The bag is fabricated from a number of specially shaped panels of durable fabric sewn together to form an elongated envelope having a generally curved configuration. The top and bottom panels of the bag include a narrowed and rearwardmost terminus positioned outside a rearward projection of either of the top or bottom panel's front edge thus allowing the distal end of the bag to be suspended directly beneath at least one of the mower handlebars. A rigid frame having a peripheral channel for engagement with brackets extending from the grass catching receptacle as well as for the receipt of a drawstring sewn into the flexible bag is provided for joining the bag and the rigid receptacle.