The assay was conducted on the northern zone of Cartago, Costa Rica, at an altitude of 2320 masl, from August to November of 1990. The objectives of the trial were: 1- to determine the type of sampling, according to the crop's age, which can be used for monitoring adult populations of Liriomyza huidobrensis (Blanchard) and 2- to observe the daily activity of L. huidobrensis throughout the day and during one crop's cycle of Solanum tuberosum. The traps used were: entomological net at 90º and 180º, count per minute, greased one gallon plastic jugs and yellows cards at three different heights. There were highly significant differ-ences among all the traps and, each one, helped to forecast particular behaviors of the flies at a given moment. The 180º net captures flies standing on or flying over the crop. The 90º net captures flies from the base to the top of the plant, helping us to forecast their vertical movement on the plant and the count per minute gives us an estimate of the number of flies on the leaves. The yellow cards help us to forecast the fly population at each stratum (high, medium and low) of the potato plant. The largest population of L. huidobrensis was found from the pre-blooming to the blooming stage with the plastic jugs and yellow cards.