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Photometry and spectra of the type Ib SN 1990I are presented and analysed,covering about 400 days of evolution. The presence of optical helium lines isshown. SN 1990I seems to show higher velocities compared to a sample of type Ibevents. The nebular emission lines display a high degree of asymmetry and thepresence of fine structures. Using the [O I] 6300,64A flux, we estimate a lowerlimit on the oxygen mass to fall in the range (0.7-1.35) M_solar. The oxygenmass requires a filling factor as small as 10^{-2} on day 254, indicating ahighly clumpy distribution of the oxygen material. A blueshift of the order 600km/s is reported in the [O I] 6300,64A after day 254. The [Ca II] 7307.5Aemission profile appears blueshifted as well at late epochs . We recover thequasi-bolometric "BVRI" light curve of SN 1990I. The constructed bolometriclight curve shows a change of slope at late phases, with an e-folding time of60 +/-2 d in the [50:200] d time interval, considerably faster than the one of{56}Co decay (i.e. 111.3 d), suggesting the gamma-rays escape with lowerdeposition, owing to the low mass nature of the ejecta. After day 200, an$e$-folding time ~47 +/-2.8 d is measured. A simplified gamma-ray depositionmodel is applied after adding a contribution of about 35% to the computedpseudo-bolometric light curves to account for near-IR luminosities to estimatethe ejecta and {56}Ni masses (M({56}Ni)=0.11 M_solar and M_{ej}=3.7 M_solar).The deficit in luminosity is estimated to be about 50% around day 308. Theobserved spectral blueshift combined with the dramatic and sudden drop in thepseudo-bolometric light curve and (B-V) colour is interpreted to be aconsequence of dust condensation in the ejecta of SN 1990I around day 250.Comment: 17 pages, 11 gigures and 5 tables. To appear in A & A (accepted

SN Ib 1990I: Clumping and dust in the ejecta?