help to correct this abstract - In this work was developed a portable fluorimeter for determination of chlorophyll a in surface water. The instrument was constructed using the following components: a microcontroller (PIC18F4550), two LEDs (light emitting diode) as light source, a multichannel photodiode (TCS-230) as detector and a digital display. LEDs with maximum at 405 and 760 nm were used. The detector TCS-230 is equipped with three optical filters inside with maximum sensitivity at 480, 540 and 690 nm besides possible use without the optical filter with high sensitivity throughout the visible spectral region with maximum sensitivity at 680 nm. With the use of LEDs and detector TCS-230 no needs of lenses, optical filter, slits and diffraction gratings in the construction of the fluorimeter. The microcontroller acts as the central processing unit and all components of the instrument controlled by him from a program previously recorded in its memory, hence no need to use a microcomputer. The fluorimeter constructed was calibrated using standard solutions of fluorescein and rhodamine 6G in HCl 1.0 x 10‑5 mol L-1 and compared with a spectrofluorimter bench top, and the results were in agreement at a level of 95%. Obtaining a linear response range for fluorescein between 0.5 and 100.0 x 10-6 and a correlation coefficient of 0.996, and for rhodamine 6G a linear response range between 0.2 and 10.0 x 10-6 and a correlation coefficient of 0.989. The instrumentation was used for determination of chlorophyll a in samples of surface water and of culture medium for algae and the results were agrees for level of 95% compared with a comparative spetrophotometric method. For chlorophyll a was obtained a concentration range between 2.5 and 1333,6 x 10-6 and a correlation coefficient of 0.995. The interval obtained serves CONAMA resolution N° 357 of 2005 that determine the classification of the quality water bodies covering the concentrations of 10.0, 30.0 and 60.0 x 10-6 g L-1 for different classes of water. The florimeter developed is inexpensive, easy to use, low power consumption allowing its use the with batteries, small size (10 cm x 20 cm x 15 cm) and with weight of 0.76 kg. The fluorimeter allows its use in determining species