The objectives of this report are to make accessible data on the abundances (1984-1987, 49dates) of sipunculans, brachiopods and hemichordates at a sand-mud flat, and on trace metals (1996,2000) and abundances (2015, 3 dates) of sipunculans and brachiopods at a sand flat in the upper Gulf ofNicoya estuary (10oN-85oW), Pacific coast of Costa Rica. Core (17.7 cm2) samples were collected at thesand-mud flat, and quadrats (0.2 m2) at the sand flat. Trace metal (Fe, Mn, Ni, Cr, Cd, Zn, and Pb)analyses were performed by Atomic Absorption Spectrometry (AAS) in tissues of the infaunal depositfeeder sipunculan Sipunculus nudus (non-depurated vs depurated) and in tissues and shells of theinfaunal filter-feeder brachiopod, Glottidia audebarti. The flats contrast in their sand (65% vs 90%) andsilt+clay (31.5% vs 5.6%) contents. At the sand-mud flat the brachiopods were represented by 129individuals, the sipunculans by 13, and the hemichordates by 185. The total area sampled by cores was1.83 m2. Estimated densities for the dates when the maximum number of each group was found, were: 29,5.7, and 40 ind./m2, respectively. Abundances were higher during the rainy season of 1984. Trace metalconcentrations in S. nudus collected at the sand flat decreased for all metals (except Cd) when medianvalues of non-depurated vs depurated individuals were compared. Maximum concentrations in non-depurated specimens, were: Fe (22.5 mg/g. dw) > Mn (234.6 µg/g. dw) > Zn (144.0 µg/g. dw) > Cu(52.7µg/g. dw) > Ni (15.5 µg/g. dw) > Cr (10.8 µg/g. dw) > Pb (9.3 µg/g. dw) > Cd (1.64 µg/g. dw).Maximum concentrations in non-depurated G. audebarti, were: Fe (21.6 mg/g. dw-soft parts) > Mn (Nodata) > Zn (123.4 µg/g. dw- soft parts) > Cu (31.4 µg/g. dw-pedicles) > Ni (No data) > Pb (21.0 µg/g.dw-shells) > Cd (5.2 µg/g. dw-soft parts) > Cr (4.7 µg/g. dw-shells). Maximum concentrations insediments were: Fe (60.3 mg/g. dw) > Mn (549.3 µg/g. dw) > Zn (83.1 µg/g. dw) > Cu (36.2 µg/g. dw)> Cr (31.5 µg/g. dw) > Pb (21.1 µg/g. dw) > Ni (20.6 µg/g. dw) > Cd (1.1 µg/g. dw).Theseconcentrations were expected for a non-industrialized estuary like the Gulf of Nicoya. At the sand flat, atotal of 76 individuals of G. audebarti, 112 of G. albida, and 366 of S. nudus were collected in 2015, in atotal sampled area of 10.6 m2. Estimated mean densities for the three dates were: 7.1, 10.5, and 31 ind./m2, for G. audebarti, G. albida, and S. nudus, respectively. The spatial distribution of the three specieswas aggregated and the maximum numbers found in a quadrat (0.2 m2) were 11, 18 and 20, respectively.The size range of G. audebarti was from 9.0 mm to 38.0 mm, and from from 6.0 mm to 29.0 mm for G.albida. The weight range of S. nudus was from 0.10 g to 2.55 g. These ranges are within those found forthese species elsewhere. Densities of G. audebarti and G. albida were relatively low, while those of S.nudus were relatively high when compared with data reported in the literature. No acorn worms werefound in 2015. G. audebarti had a relatively stable abundance, while G. albida decresed at the end of therainy season of 2015. The abundance of S. nudus increased towards the end of the rainy season andheavier (more than 1.25 g) specimens were also collected at this time. A strong ENSO was fullydeveloped in 2015 leading to above normal average temperatures and less rainfall in the Pacific coast ofCosta Rica, but the impact of this warming event on the fauna is unknown.