By Rebecca Cook WHEN I FIRST moved to Tucson, seasoned residents gave me the lowdown on eating Mexican food. Most importantly, I had to understand that the best Mexican food in these United States was to be found in Tucson, no matter what I might hear from displaced and disgruntled Texans. And second, for the best Mexican food money could buy, I was to head south of 22nd Street. I've found the first bit of wisdom to be undeniably true. The second piece of advice I've had reason to question over the years. While many of my favorite Mexican restaurants are on the southside, I've found some glorious exceptions. Rosa's Mexican Food has dished out quality food in Tucson for more than 25 years, first at a snug location near Speedway and Campbell and, more recently, at Campbell Avenue and Fort Lowell Road. Two recent visits demonstrated the good food and crisp service are by no means secret: On both occasions the place was packed nearly to capacity. Owned and operated by the Ortega family, Rosa's is a restaurant that understands its clientele and works hard to meet its needs. When we arrived just after 11 a.m., the restaurant was set up for a workday crowd--every table was equipped with menus, water, chips and salsa. Within 10 minutes of sitting down, we had food and drink laid before us and could enjoy it all, knowing we'd be back to work within our one-hour lunch time. Being able to get in and out quickly isn't the only reason Rosa's is so popular: It's the food that keeps people coming back. In addition to a complete menu of Mexican food specialties, Rosa's features several daily specials, which can include such tempting dishes as carne asada, posole or fajitas. On my first visit, I ordered the carne seca chimichanga enchilada-style, while my companion (who just happens to be from Texas) ordered the green chile burro. The chimichanga was fully stuffed and served with a savory enchilada sauce. Although delicious, the carne seca reminded me more of machaca, a similar beef preparation also found on the menus of many Mexican restaurants. At any rate, the beef was tender and deeply satisfying despite my musings on whether it should more properly be called something else. The green chile was tasty with tender chunks of beef served in a cream-colored green chile sauce permeated with garlic. Side dishes at Rosa's often include Spanish rice and refried beans. At first I thought the beans were a little bland, but after asking how they were prepared and learning they're made with vegetable oil, I felt duly chastised. The flavor I was no doubt missing was rendered by lard, something not at all acceptable in this era. Our second visit occurred on a Sunday evening, which I never imagined would be busy. But at Rosa's every table was occupied and people streamed in to get their take-out orders. While not as speedy as lunch, the service was still sprightly, and we managed to be in and out in just over an hour. This time my Texas friend tried the chicken fajitas, one of the specials of the day, while I had the standing Sunday special with chile relleno, bean tostada and chicken enchilada. The chicken fajitas were incredible, with grilled strips of chicken done to perfection and slices of green and red peppers, onions and minced garlic. Served with a pizza-pan-sized flour tortilla, this was a dish to savor and enjoy. The chile relleno came with a light batter coating that was slightly soggy, although the chile was soft and generously stuffed with yellow cheese. The chicken enchilada was yummy enough to make me wish I'd ordered a whole platter. Rosa's enchilada sauce this time covered a balanced blend of corn tortilla, cheese and tender, moist chicken. The bean tostada was also good, covered with shredded cheese, lettuce and tomato. Rosa's salsa, so necessary with those chips, and a delicious addition to all other dishes, is a puree of tomatoes, chiles and spices, the most noticeable of which is garlic. If you live anywhere near Campbell and Fort Lowell, and you're craving good Mexican food but don't want to spend a half hour in the car, don't despair. Rosa's serves up some of the best stuff around. Even my friend from Texas begrudgingly agrees.

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