On May 5, 2009, at 1741 Pacific daylight time, a Ayres S2R-T34, N40208, and a Grumman G164B, N6617K, collided while landing on runway 23 at Yuba County Airport, Marysville, California. GN Diddle, Inc., operated the Ayres S2R, and Onstott Dusters operated the Grumman G-164B. Both were operating under the provisions of Title 14 of the Code of Federal Regulations Part 137. The commercial pilot of the Ayres S2R was not injured, and the commercial pilot of the Grumman G-164B was seriously injured. Both airplanes were substantially damaged. Visual meteorological conditions prevailed, and no flight plans had been filed. The pilot of the Ayres S2R stated in his NTSB Pilot Accident Report that he was at 500-600 feet above ground level (agl), entered left base, and then turned on to a 3/4-mile final for runway 23. About 30 feet above the ground he impacted with something underneath him, and the airplane fell to the runway. After the crash he realized that his airplane and another airplane had come together over the runway. At no time, prior to the collision, did he have visual contact with the other airplane. The pilot of the Grumman G-164B stated in his NTSB Pilot Accident Report that he entered a straight in for runway 23, looked for traffic and did not see any. He was over the runway numbers, preparing to land, when he violently lost control of the airplane and hit the ground. As he was being helped out of his airplane he realized his airplane had been struck from behind by another airplane. Yuba County Airport is an uncontrolled airport. Traffic is coordinated between aircraft using a common traffic advisory frequency (CTAF), and pilot visual lookout. Neither airplane was equipped with a VHF radio.