Data on the abundances and metal concentrations in intertidal estuarine macro-invertebrates from the Eastern Tropical Pacific are rare. Thus, the objectives of this report are to make accessible data on the abundances (1984-1987, 49 dates; 2013, 12 dates) of sipunculans (Sipuncula), brachiopods (Brachiopoda) and hemichordates (Enteropneusta) 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 of Nicoya estuary (10 oN - 85 oW), Pacific coast of Costa Rica. Core (17.7 cm2) samples were collected at the sand-mud flat, and quadrats (0.2 m2) at the sand flat. The flats contrasted (1984 vs 2015) mainly in their sand (65 % vs 90 %) silt+clay (31.5 % vs 5.6 %) and organic matter (2 vs 3 %) contents. The 1984-1987 survey yielded 13 sipunculans (12 Sipunculus nudus, 1 Apionsoma. trichocephalus) 129 brachiopods (Glottidia spp.) and 185 hemichordates (acorn worms). The total area sampled by cores was 1.83 m2. Estimated densities for the dates when the maximum number of each group was found, were: 5.7, 29, and 40 ind./m2, respectively. Abundances were higher during the rainy season of 1984. Strong red tides in the Gulf of Nicoya during 1985 may have influenced the abundances. Trace metal (Fe, Mn, Ni, Cr, Cd, Zn, and Pb) analysis was performed by Atomic Absorption Spectrometry (AAS) in tissues of the infaunal deposit feeder S. nudus (non-depurated vs depurated) and in tissues and shells of the infaunal filter-feeder lingulide brachiopod, G. audebarti. Maximum mean concentrations of metals in S. nudus worms found either during the 1996 or 2000 surveys, were: For non-depurated worms, Fe (16.0 mg/g dw) > Mn (165 µg/g dw) > Zn (81 µg/g dw) > Cu (26 µg/g dw) > Cr (11 µg/g dw) > Ni (10.4 µg/g dw) > Pb (9.3 µg/g dw) > Cd (1.2 µg/g dw). For depurated worms: Fe (5.0 mg/g dw) > Mn (61 µg/g dw) > Zn (39 µg/g dw) > Cu (24 µg/g dw) > Ni (8.4 µg/g dw) > Pb (2.7 µg/g dw) > Cd (0.62 µg/g dw). However, median concentrations in non-depurated vs non-depurated S. nudus were not statistically significant. For G. audebarti (1996 only): Fe (1.6 mg/g dw-soft parts) > Zn (123.4 µg/g dw-soft parts) > Cu (31.4 µg/g dw-pedicles) > Pb (21.0 µg/g dw-shells) > Cd (5.2 µg/g dw-soft parts) > Cr (4.7 µg/g dw-shells). For sediments; Fe (46 mg/g dw) > Mn (41.3 µg/g dw) > Zn (63 µg/g dw) > Cu (36.2 µg/g dw) > Cr (31.5 µg/g dw) > Pb (21.1 µg/g dw) > Ni (16.1 µg/g dw) > Cd (1.1 µg/g dw). These concentrations were expected for a non-industrialized estuary like the Gulf of Nicoya. During the 2013 surveys of the Punta Morales flat the percentages of sand (40.5 %), silt + clay (57.6 %) and organic matter (8 %) were different from those of 1984. Seven sipunculans (A. trichocephaus) 16 acorn worms and no brachiopods were found. At the Cocorocas sand flat, a total of 76 individuals of G. audebarti, 112 of G. albida, and 366 of S. nudus were collected in 2015, in a total sample 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 species was aggregated and the maximum number of individuals found in a quadrat (0.2 m2) was 11, 18 and 20, respectively. The size range of G. audebarti was from 9.0 mm to 38.0 mm, and 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 were within those found for these 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 were found at the sand flat. G. audebarti had a relatively stable abundance, while G. albida decreased at the end of the rainy season of 2015. The abundance of S. nudus increased towards the end of the rainy season and heavier (more than 1.25 g) specimens were also collected at this time. A strong ENSO was fully developed in 2015 leading to above normal average temperatures and less rainfall in the Pacific coast of Costa Rica, but the impact of this warming event on the fauna is yet unknown.