Abstract: The role of magne-c ﬁeld in the AGN jet physics is s-ll not fully determined. At pc scale, it is known that it is important in the accelera-on and collima-on processes while at arcsecond scale it could reveal fundamental pieces of the jet dynamics and energe-cs and its surrounding environment. At intermediate scales, the scenario is more debated. To contribute in this framework, we need to resolve polarized emission even in the low surface brightness extended structures (e.g. lobes). This absolutely requires high sensi-vity observa-ons. With the advent of ALMA, now it is possible also in the millimeter, a band which was unexplored by previous facili-es. Here I present the impressive images in polariza-on obtained using ALMA archival mul- band data of an ALMA calibrator PKS 0521-365 which represents a prototype of BL Lac object with extended resolved structures (jet and hotspot) at all frequencies from op-cal to X-rays. Polarized emission in the mm band of PKS0521-365: ALMA observaAons. E.Liuzzo, R. Paladino, V. Galluzzi & IT ARC node Next steps and perspecAves: ALMA observaAons and results The peculiar case of PKS 0521-365 References: This is a nearby (z = 0.0554) radio-loud object and bright FERMI source, exhibi-ng a variety of nuclear and extranuclear phenomena (Falomo et al. 2009). It is one of the most remarkable object in the sourthern sky: It is one of the three known BL Lac objects showing a kiloparsec-scale jet well resolved at all bands (Liuzzo et al. 2011). As showed in Fig.1, a one-side radio jet extends in N-W side up to 7 arcsec , with the presence of many knots that are also detected from op-cal to X–rays (Falomo et al. 2009). An hotspot is also detected in all bands at 8 arcsec from the nucleus in the southeast direc-on. At low frequency, the arcsecond-scale radio structure is dominated by an extended lobe. The overall energy distribu-on of PKS 0521-365 is consistent with a jet oriented at about 30 degrees with respect to the line of sight. This is also in agreement with the absence of superluminal mo-on in the parsec–scale jet (Falomo et al. 2009). In the millimeter bands, extended structures (hotspot and jet) of this object are detected up to 320 GHz, with similar structures from op-cal to X–rays (Liuzzo et al. 2015, Leon et al 2016). An es-mate of molecular gas content is also given together with an analysis od the SED of each source component (Liuzzo et al. 2015). We analysed polariza-on data in • Band 3 from our propretary data (PI: V. Galluzzi); • Band 5 from science veriﬁca-on observa-ons; • Band 7 from public archival data. We found that the source is well resolved upt to 350 GHz with detec-on of core, inner jet and hotspot. Polarized emission is revealed: • in the core and hotspot up to B 7. • also in the extended jet and lobe in B 3 with posi-on angle parallel to the jet direc-on and to the shock front in the lobe. Brentjens & de Bruyn 2005, A&A, 441 1217, Falomo et al. 2009, A&A, 501, 907 Leon et al. 2016, A&A, 586, A70 Liuzzo et al. 2011, arXiv:1105.5226 Liuzzo et al. 2015 Revolu-on in Astronomy with ALMA: the 3° year, 499, 129 In the following I discuss what it is now possible to do thanks the presented high quality ALMA data in polariza-on, like those presented here: • Magne&c ﬁeld study in radio loud AGN: Nearly 10-20 % of AGN are radio loud and the form of their spectra as a func-on of frequency implies that the radio emission is non-thermal (synchrotron) in origin, due to rela-vis-c plasma moving in strong and ordered magne-c ﬁelds. Since the polarised signal in these objects is typically a few percentage of the total intensity, collec-ng informa-on on magne-c ﬁeld of RL AGN requires telescope with high sensi-vity (<<1mJy/beam) as ALMA. The results obtained for PKS 0521-365 show that in only 10 mins on source polarized emission is revealed even in the lobes with angular resolu-on < 0.5 arcsec, demonstra-ng that now with ALMA impressive results could be reached also for faint source com