On February 20, 1995, at 1310 Pacific standard time, a Grumman AA5B, N28003, and a Citabria 7ECA, N9082L, collided during cruise flight near Pacifica, California. Visual meteorological conditions prevailed. There were no injuries to the private pilot and passenger flying in N28003, or to the private pilot flying in N9082L. Both pilots reported that at the time of the accident they were making personal flights. In pertinent part, the Grumman pilot reported to the National Transportation Safety Board that he had taken off from the Half Moon Bay Airport at 1255, and he was proceeding in a northward direction toward San Francisco. After climbing to about 1,300 feet mean sea level, he observed the (slower flying) Citabria ahead of him. On the UNICOM frequency, he informed the Citabria pilot that he planned to pass to the Citabria's left side, and he heard the Citabria pilot acknowledge his transmission. As the Grumman pilot was overtaking and nearly abeam the Citabria, the Grumman pilot observed a southerly flying twin-engine airplane approaching nearly head-on. The Grumman pilot indicated to the Safety Board that he watched the approaching traffic and briefly rolled his airplane's wings left and right. He then lost sight of the Citabria, felt a vibration, and realized a collision had occurred. The Citabria pilot reported to the Safety Board that he had taken off from the Half Moon Bay Airport at 1305, and he was proceeding in a northward direction toward Novato, California. After climbing to about 1,320 feet mean sea level, he heard the pilot flying the Grumman report being at his 8 o'clock position. The Grumman pilot indicated that he planned to overtake and pass the Citabria on the Citabria's left side. The Citabria pilot further reported that he held his altitude and heading steady. He looked out his left window and observed the Grumman in his 8 o'clock position. Seconds later, the Grumman was in his 9 o'clock position. The Citabria pilot stated that he then observed a twin-engine airplane in his 11 o'clock position flying in a southerly direction. The Citabria pilot further reported that he did not need to make any turns to avoid colliding with the twin-engine airplane which appeared to be slightly above his altitude. Then, according to the Citabria pilot, he "felt a bump" and saw the Grumman above him and to his left. The Citabria pilot also reported that he then observed that his airplane's left wing had ". . . started to come apart from the wing tip to the cabin." Both airplanes returned to the Half Moon Bay Airport and landed without further mishap. The Citabria pilot reported that the Grumman's propeller had evidently sliced through his airplane's left wing thus partially severing its rear spar and aileron (see attached wreckage diagram). Regarding flight visibility, neither pilot reported experiencing any difficulty in observing each other's airplane. Both pilots reported that their forward flight visibility was restricted by haze.